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

* Semi-automated using https://github.com/myii/ssf-formula/pull/27
* Fix errors shown below:

```bash
nginx-formula$ yamllint -s .
./pillar.example
  146:56    error    empty value in block mapping  (empty-values)
  147:76    error    empty value in block mapping  (empty-values)
  148:59    error    empty value in block mapping  (empty-values)
  150:22    error    empty value in block mapping  (empty-values)
  151:39    error    empty value in block mapping  (empty-values)
  152:39    error    empty value in block mapping  (empty-values)
  153:39    error    empty value in block mapping  (empty-values)
```

.travis.yml

       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/default/pillar/nginx.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

 
 # 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

               - hostname2.example.com:1000
         upstream stream_backend:
           least_conn: ''
-          'server backend1.example.com:12345 weight=5':
-          'server backend2.example.com:12345 max_fails=2 fail_timeout=30s':
-          'server backend3.example.com:12345 max_conns=3':
+          'server backend1.example.com:12345 weight=5': ~
+          'server backend2.example.com:12345 max_fails=2 fail_timeout=30s': ~
+          'server backend3.example.com:12345 max_conns=3': ~
         upstream dns_servers:
-          least_conn:
-          'server 192.168.136.130:53':
-          'server 192.168.136.131:53':
-          'server 192.168.136.132:53':
+          least_conn: ''
+          'server 192.168.136.130:53': ~
+          'server 192.168.136.131:53': ~
+          'server 192.168.136.132:53': ~
         server:
           listen: 1000
           proxy_pass: lb-1000