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.
Fixed some CS and phpdoc
Use the autoloader for test files rather than requiring them by hand
Fixed coding standards in tests
Made a phpdoc more specific for the return value
Removed duplication of the parsing of table rows
Reduced the cyclomatic complexity of the ArrayLoader
Updated the badges displayed in the readme
Merge pull request #75 from stof/badges

Updated the badges displayed in the readme
Merge pull request #74 from stof/reduce_array_loader_complexity

Reduced the cyclomatic complexity of the ArrayLoader
Merge pull request #73 from stof/parser_duplication

Removed duplication of the parsing of table rows
Merge pull request #70 from stof/refactor_yaml_loader

Refactored the YamlFileLoader to remove duplication
Updated the contribution guidelines
Merge pull request #76 from stof/contribution_guidelines

Updated the contribution guidelines
Removed duplication from the ArrayLoader