Travis CI

Added Travis CI so that syntax errors can be caught at the Pull Request stage before they hitting the master branch.  It also checks whether switching on `$o-colors-is-silent` really silences the output.

**Some other small fixes:**
- Sass partials should be prefixed with underscores; only compile-able files should not be.
- No need to add underscores to `@import`s.
- `$o-colors-is-silent`'s logic was the wrong way round :-) (regression test added).
- In `.gitignore`: only match `/.sass-cache/` named folders in repo root; added `/build/` (and `*.swp` for vim :sunglasses:)

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Before merging this Pull Request please switch Travis CI on for this project.  To be able to switch on Travis CI for o-colors a manager of the **o-color** will need to sign up for Travis and go here: https://travis-ci.org/profile, click **Financial Times** and set **o-color** to **ON**.  Alternatively if someone could grant me admin access I can do this.