closes #57

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

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

./logrotate/osfamilymap.yaml
  1:1       warning  missing document start "---"  (document-start)
  7:15      warning  truthy value should be one of [false, true]  (truthy)
  12:14     warning  truthy value should be one of [false, true]  (truthy)
  14:23     warning  truthy value should be one of [false, true]  (truthy)
  16:14     warning  truthy value should be one of [false, true]  (truthy)
  24:14     warning  truthy value should be one of [false, true]  (truthy)

./logrotate/defaults.yaml
  4:1       warning  missing document start "---"  (document-start)
  11:23     warning  truthy value should be one of [false, true]  (truthy)
  13:13     warning  truthy value should be one of [false, true]  (truthy)
  15:13     warning  truthy value should be one of [false, true]  (truthy)
  16:1      error    too many blank lines (1 > 0)  (empty-lines)

pillar.example
  3:1       warning  missing document start "---"  (document-start)
  5:4       warning  missing starting space in comment  (comments)
  6:6       warning  missing starting space in comment  (comments)
  6:5       warning  comment not indented like content  (comments-indentation)
  7:6       warning  missing starting space in comment  (comments)
  9:13      warning  truthy value should be one of [false, true]  (truthy)
  11:13     warning  truthy value should be one of [false, true]  (truthy)
  12:15     warning  truthy value should be one of [false, true]  (truthy)
  13:14     warning  truthy value should be one of [false, true]  (truthy)
  26:12     error    trailing spaces  (trailing-spaces)
  48:12     error    too many spaces after hyphen  (hyphens)
  69:1      error    too many blank lines (1 > 0)  (empty-lines)
```