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

@stof This is going to be default behavior and lots of PHP projects do it already — they `self-update` before they `install`.

Imagine your current `.travis.yml` is this:

```
language: php
php:
  - 5.5
  - hhvm

before_install: composer self-update

install: composer install

script: phpunit
```

This PR allows you to trim it down to:

```
language: php
php:
  - 5.5
  - hhvm
```

Less to maintain.