travis test matrix against multiple tmux versions

Update ``.travis.yml`` config allows testing against multiple versions of tmux using [build matrix](http://about.travis-ci.org/docs/user/build-configuration/#The-Build-Matrix).

Preview: https://travis-ci.org/tony/tmuxinator/builds/12659191.

It is currently set to test versions 1.5 - 1.8 and the latest codebase.

Can be configured to 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.

I made another ticket for this on remiprev/teamocil#51 (a similar project) since both use travis and tmux.