Improved the Travis build matrix

Tests again the different PHP versions are now resolving the composer
dependencies unmodified. Tests again other Symfony versions are runing
only for PHP 5.5 to limit the number of jobs in the matrix.
Testing against PHP 5.6 and against Symfony 2.5@dev has been added.
Merge pull request #55 from stof/simplify_travis

Improved the Travis build matrix