Simplified the Travis build matrix

Testing against older symfony versions is now done only for 1 version of
PHP rather than all of them.
Merge pull request #71 from stof/simplify_travis_matrix

Simplified the Travis build matrix
Refactored the YamlFileLoader to remove duplication

The YamlFileLoader now uses the ArrayLoader by composition rather than
inheritance, allowing to extend from the AbstractFileLoader, which is way
more logical.
Changed the parsing of Yaml to pas the file content for forward compat

Passing a filename as input to Yaml::parse is deprecated.