Simplified the travis build matrix

Given that the extension does not rely on lots of Symfony code by itself,
the testing against multiple Symfony version is not done for all PHP
versions anymore:

- Symfony 2.2.* is not tested anymore (it is an EOL version)
- builds are using the dependencies resolved by Composer (latest stable
  components, so 2.4.* at this time)
- extra jobs are added in the matrix to test the extension with Symfony
  2.3.* and 2.5.*@dev on a single PHP version

This means that the matrix goes from 17 jobs to 7, making it much faster
(we can only have 5 concurrent jobs for the whole github organization).
Merge pull request #128 from stof/simplify_build_matrix

Simplified the travis build matrix