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

I think most projects have it because in the early days of composer, running self-update was necessary to ensure running a version with bug fixes included, without waiting for Travis to update. But composer is much more stable than 2 years ago so it is not really necessary anymore, except projects have not removed it from their config (and other newer projects have continued to copy-paste the config they found in older projects).

I agree to allow trimming it. What I'm sayign is that ``composer self-update`` is not needed in the trimmed version.

Btw, it may be worth change the default PHP version to something newer than 5.3, which is EOL