Update .travis.yml

Added PHP 5.5, 5.6 and HHVM.
Updated handling of composer.
PHPUnit is now outputting the coverage as text.
Merge pull request #4 from h4cc/patch-1

Update .travis.yml
Ignore composer self-update failures.
Merge pull request #5 from h4cc/patch-2

Ignore composer self-update failures.