Enhancement: allows install to be performed through checked in composer....

...phar

 * re-activates automatic composer install on travis-ci
 * supports running install with a checked in composer.phar
 * doesn't self-update the checked in composer.phar

Wrote blog post for updates:

 * http://till.klampaeckel.de/blog/archives/204-Whats-wrong-with-composer-and-your-.travis.yml.html

Write tests for:

 * [x] `composer self-update` is run
 * [x] `composer install` is run
 * [x] `composer.phar install` is run
 * [ ] `composer self-update`is not run
 * [x] `composer_args` are used

cc @joshk – probably shouldn't happen in this PR? But all in all I think it would make sense to set the default PHP to 5.4 at least. But that requires another announcement as builds might fail.