Makes library fully compatible with symfony components version [2|3|4] and PHP 5.3-7.2

[xthiago]

Now the library is fully compatible with:

• PHP 5.3, 5.4, 5.5, 5.6, 7.0, 7.1 and 7.2
• Symfony 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 3.0, 3.1, 3.2, 3.3, 3.4 and 4.0

I have updated the .travis in order to ensure these settings on CI. Result: https://travis-ci.org/xthiago/pdf-version-converter/builds/340779966