ci(yamllint): add rule empty-values & use new yaml-files setting

* Semi-automated using myii/ssf-formula#27
* Fix errors shown below:

```bash
vim-formula$ yamllint -s .
./pillar.example
  21:8      error    empty value in block mapping  (empty-values)
  22:9      error    empty value in block mapping  (empty-values)
  23:8      error    empty value in block mapping  (empty-values)
  25:13     error    empty value in block mapping  (empty-values)
  26:15     error    empty value in block mapping  (empty-values)
  27:15     error    empty value in block mapping  (empty-values)
  28:15     error    empty value in block mapping  (empty-values)
  29:15     error    empty value in block mapping  (empty-values)
  30:12     error    empty value in block mapping  (empty-values)
  32:12     error    empty value in block mapping  (empty-values)
  34:18     error    empty value in block mapping  (empty-values)
  35:18     error    empty value in block mapping  (empty-values)
  37:8      error    empty value in block mapping  (empty-values)
  38:8      error    empty value in block mapping  (empty-values)
  39:13     error    empty value in block mapping  (empty-values)
  43:12     error    empty value in block mapping  (empty-values)

./test/salt/pillar/vim.sls
  7:12      error    empty value in block mapping  (empty-values)
```

.travis.yml

@@ -56,9 +56,9 @@ jobs:
       before_install: skip
       script:
         # Install and run `yamllint`
-        - pip install --user yamllint
-        # yamllint disable-line rule:line-length
-        - yamllint -s . .yamllint pillar.example test/salt/pillar/vim.sls
+        # Need at least `v1.17.0` for the `yaml-files` setting
+        - pip install --user yamllint>=1.17.0
+        - yamllint -s .
         # Install and run `commitlint`
         - npm install @commitlint/config-conventional -D
         - npm install @commitlint/travis-cli -D

.yamllint

@@ -6,10 +6,24 @@ extends: default
 
 # Files to ignore completely
 # 1. All YAML files under directory `node_modules/`, introduced during the Travis run
+# 2. Any SLS files under directory `test/`, which are actually state files
 ignore: |
   node_modules/
+  test/**/states/**/*.sls
+
+yaml-files:
+  # Default settings
+  - '*.yaml'
+  - '*.yml'
+  - .yamllint
+  # SaltStack Formulas additional settings
+  - '*.example'
+  - test/**/*.sls
 
 rules:
+  empty-values:
+    forbid-in-block-mappings: true
+    forbid-in-flow-mappings: true
   line-length:
     # Increase from default of `80`
     # Based on https://github.com/PyCQA/flake8-bugbear#opinionated-warnings (`B950`)

pillar.example

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
 ---
 vim:
   managed_vimrc: true
@@ -18,29 +20,29 @@ vim:
         /srv/*.map: 'set filetype=sls'
 
   settings:
-    et:
-    cin:
-    ru:
+    et: ~
+    cin: ~
+    ru: ~
     bs: indent,eol,start
-    showcmd:
-    showmatch:
-    smartcase:
-    incsearch:
-    autowrite:
-    hidden:
+    showcmd: ~
+    showmatch: ~
+    smartcase: ~
+    incsearch: ~
+    autowrite: ~
+    hidden: ~
     mouse: a
-    number:
+    number: ~
     backspace: 2
-    noerrorbells:
-    novisualbell:
+    noerrorbells: ~
+    novisualbell: ~
     background: dark
-    ai:
-    si:
-    cindent:
+    ai: ~
+    si: ~
+    cindent: ~
     tabstop: 8
     softtabstop: 8
     shiftwidth: 8
-    nowrap:
+    nowrap: ~
     swapsync: ''
 
   mappings:

test/salt/pillar/vim.sls

@@ -1,7 +1,9 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
 ---
 vim:
   managed_vimrc: true
   config:
     syntax: 'on'
   settings:
-    number:
+    number: ~