support for symfony >= 2.3 & 3.0

[mimmi20]

• drop support for symfony 2.1
• add support for symfony >= 2.3 & 3.0
• update travis config
• fix tests
• use 2.x browscap/browscap-php lib - I now is deprecated but it is not case of this pr

This PR replaces PR#17.

Aditional to the original PR, some coding style issues were fixed

Info: It looks like the bundle is incompatible with symfony 4.