feat(yamllint): include for this repo and apply rules throughout
* Semi-automated using `ssf-formula` (v0.5.0)
* Fix errors shown below:
```bash
apt-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
pillar.example
7:1 error trailing spaces (trailing-spaces)
test/salt/pillar/repositories.pillar.sls
1:1 warning missing document start "---" (document-start)
19:1 error too many blank lines (1 > 0) (empty-lines)
test/salt/pillar/preferences.pillar.sls
1:1 warning missing document start "---" (document-start)
```
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