Run composer install in Travis CI before tests

Tests are using the Composer autoloader since: https://github.com/erusev/parsedown/commit/cd1c030362351d6006f0c2cbe52b7fbd65ca9ef8

Because of that Composer should have actually dumped the autoloader in the `vendor/` folder,
before the tests are ran.
Merge pull request #158 from hkdobrev/travis-composer

Run composer install in Travis CI before tests