Travis to test multiple version of tmux

Howdy,

Testing of teamocil on multiple versions of tmux using [build matrix](http://about.travis-ci.org/docs/user/build-configuration/#The-Build-Matrix).

Here is a preview of the test matrix: https://travis-ci.org/tony/teamocil/builds/12658860

It is currently set to test versions 1.5 - 1.8 and the latest codebase.

You can allow versions to be tested and fail optionally with:

```yaml
matrix:
  allow_failures:
    - env: TMUX_VERSION=1.5
```

1.5 could be any ``env`` variable set.