Add Caching To Travis Build

[alexfinnarn]

Adding caching should speed up the builds.

cache:
  directories:
    - $HOME/.composer/cache/files

References:

• https://blog.wyrihaximus.net/2015/07/composer-cache-on-travis/
• https://docs.travis-ci.com/user/caching/

[mradcliffe]

Ooh, thanks. I didn't know about this feature and it makes a lot of sense.

I think it makes sense to add --prefer-dist in the travis install section.