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

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

postfix-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
./postfix/osfamilymap.yaml
  1:1       warning  missing document start "---"  (document-start)

./postfix/services.yaml
  11:1      warning  missing document start "---"  (document-start)
  12:1      warning  comment not indented like content  (comments-indentation)
  221:1     warning  comment not indented like content  (comments-indentation)

./postfix/defaults.yaml
  4:1       warning  missing document start "---"  (document-start)

pillar.example
  1:1       warning  missing document start "---"  (document-start)
  2:25      warning  truthy value should be one of [false, true]  (truthy)
  13:17     warning  truthy value should be one of [false, true]  (truthy)
  16:17     warning  truthy value should be one of [false, true]  (truthy)
  22:17     warning  truthy value should be one of [false, true]  (truthy)
  23:17     warning  truthy value should be one of [false, true]  (truthy)
  25:17     warning  truthy value should be one of [false, true]  (truthy)
  29:17     warning  truthy value should be one of [false, true]  (truthy)
  31:17     warning  truthy value should be one of [false, true]  (truthy)
  35:17     warning  truthy value should be one of [false, true]  (truthy)
  47:15     warning  truthy value should be one of [false, true]  (truthy)
  50:18     warning  truthy value should be one of [false, true]  (truthy)
  53:21     warning  truthy value should be one of [false, true]  (truthy)
  62:24     warning  truthy value should be one of [false, true]  (truthy)
  66:31     warning  truthy value should be one of [false, true]  (truthy)
  69:19     warning  truthy value should be one of [false, true]  (truthy)
  70:19     warning  truthy value should be one of [false, true]  (truthy)
  73:14     warning  truthy value should be one of [false, true]  (truthy)
  74:21     warning  truthy value should be one of [false, true]  (truthy)
  78:14     warning  truthy value should be one of [false, true]  (truthy)
  89:15     error    trailing spaces  (trailing-spaces)
  96:6      warning  missing starting space in comment  (comments)
  130:89    error    line too long (105 > 88 characters)  (line-length)
  131:89    error    line too long (101 > 88 characters)  (line-length)
  152:5     error    duplication of key "sender_canonical_maps" in mapping  (key-duplicates)
  154:5     error    duplication of key "virtual_alias_maps" in mapping  (key-duplicates)
  190:5     error    duplication of key "use_file" in mapping  (key-duplicates)
  235:9     error    wrong indentation: expected 10 but found 8  (indentation)
  248:1     error    duplication of key "postfix" in mapping  (key-duplicates)
  258:85    error    trailing spaces  (trailing-spaces)

Refer back to: https://github.com/saltstack-formulas/template-formula/pull/159#issue-305203039.

saltstack-formulas / postfix-formula

feat(yamllint): include for this repo and apply rules throughout #95