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
postfix-formula$ yamllint -s .
./pillar.example
  92:15     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/default.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
   key-duplicates:
     ignore: |
       pillar.example

pillar.example

@@ -89,7 +89,7 @@ postfix:
     readme_directory: 'no'
     myhostname: localhost
     mydestination: localhost, localhost.localdomain
-    relayhost:
+    relayhost: ''
     mynetworks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
     mailbox_size_limit: 0
     recipient_delimiter: +