feat(yamllint): include for this repo and apply rules throughout

* Semi-automated using `ssf-formula` (v0.5.0)
* Fix (or ignore) errors shown below:

```bash
postgres-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
./postgres/repo.yaml
  4:2       error    syntax error: found character '%' that cannot start any token
  6:89      error    line too long (110 > 88 characters)  (line-length)
  9:89      error    line too long (95 > 88 characters)  (line-length)
  13:89     error    line too long (104 > 88 characters)  (line-length)
  14:89     error    line too long (106 > 88 characters)  (line-length)

./postgres/defaults.yaml
  3:1       warning  missing document start "---"  (document-start)
  4:22      warning  truthy value should be one of [false, true]  (truthy)
  5:16      warning  truthy value should be one of [false, true]  (truthy)
  5:22      warning  too few spaces before comment  (comments)
  20:10     warning  truthy value should be one of [false, true]  (truthy)
  40:8      warning  missing starting space in comment  (comments)
  41:89     error    line too long (97 > 88 characters)  (line-length)
  71:11     error    too many spaces after colon  (colons)
  72:12     warning  truthy value should be one of [false, true]  (truthy)
  74:15     warning  truthy value should be one of [false, true]  (truthy)
  85:6      warning  missing starting space in comment  (comments)
  89:11     warning  truthy value should be one of [false, true]  (truthy)
  90:24     warning  truthy value should be one of [false, true]  (truthy)
  91:56     error    too few spaces after comma  (commas)

./postgres/codenamemap.yaml
  3:2       error    syntax error: found character '%' that cannot start any token
  8:89      error    line too long (98 > 88 characters)  (line-length)
  49:89     error    line too long (110 > 88 characters)  (line-length)
  53:89     error    line too long (99 > 88 characters)  (line-length)

./postgres/osmap.yaml
  1:2       error    syntax error: found character '%' that cannot start any token
  5:89      error    line too long (115 > 88 characters)  (line-length)

./postgres/osfamilymap.yaml
  1:2       error    syntax error: found character '%' that cannot start any token
  57:89     error    line too long (90 > 88 characters)  (line-length)
  58:89     error    line too long (113 > 88 characters)  (line-length)
  134:89    error    line too long (103 > 88 characters)  (line-length)
  135:89    error    line too long (127 > 88 characters)  (line-length)
  205:89    error    line too long (91 > 88 characters)  (line-length)
  206:89    error    line too long (91 > 88 characters)  (line-length)

pillar.example
  3:1       warning  missing document start "---"  (document-start)
  8:22      warning  truthy value should be one of [false, true]  (truthy)
  13:16     warning  truthy value should be one of [false, true]  (truthy)
  20:4      warning  missing starting space in comment  (comments)
  21:4      warning  missing starting space in comment  (comments)
  42:4      warning  missing starting space in comment  (comments)
  65:4      warning  missing starting space in comment  (comments)
  87:4      error    syntax error: found character '%' that cannot start any token

test/salt/pillar/postgres.sls
  4:2       error    syntax error: found character '%' that cannot start any token
```

.yamllint

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
+---
+# Extend the `default` configuration provided by `yamllint`
+extends: default
+
+# Files to ignore completely
+# 1. All YAML files under directory `node_modules/`, introduced during the Travis run
+# 2. All YAML files heavily reliant on Jinja; these can be tackled in a subsequent PR
+ignore: |
+  node_modules/
+  pillar.example
+  postgres/codenamemap.yaml
+  postgres/osfamilymap.yaml
+  postgres/osmap.yaml
+  postgres/repo.yaml
+  test/salt/pillar/postgres.sls
+
+rules:
+  line-length:
+    # Increase from default of `80`
+    # Based on https://github.com/PyCQA/flake8-bugbear#opinionated-warnings (`B950`)
+    max: 88