Make sure we test with multiple symfony versions

It is two fixes in this commit. 

## Fix 1

Make sure we test with multiple versions of Symfony. At the moment we are just checking with the latest version.

This is a run with [SYMFONY_VERSION=2.0.16](https://travis-ci.org/ekino/EkinoNewRelicBundle/jobs/16181133). But as you can see, composer downloads symfony/framework-bundle v2.4.0.

I removed *before_script* and replaced it with a call to the built-in Composer and I'm making sure we use the SYMFONY_VERSION variable...

Here is the [Travis documentation](http://about.travis-ci.org/docs/user/languages/php/) where you can find examples of using variables.

## Fix 2

In composer.json it is specified that we support Symfony2.2 and later. There is no need of testing Travis with Symfony2.0.16. You should change one or the other, I chosed to change .travis.yml since composer.json seems to be more entertained. 

~~I changed the max version to the latest stable release of Symfony, because that is the max version composer.json lets us use. If we want to check with origin/master we should add *@dev* on *symfony/framework-bundle* in the composer file.~~