A few changes to make the rest-proxy more usable as a library

I have drafted a few changes that help me using rest-proxy as a library in combination with a few other libraries.

The first patch downgrades requirements to more stable branches of the respective symfony components. I'm not 100% sure about the implications but considering how little of the symfony is actually used in the code base, I don't foresee any issues. I have briefly tested the code, so far no problems encountered.

The second patch proposes a visibilty change of some components. The most important part is the visibility of doMethod() and decodeOut(), as it's impossible to override them which is a key part of extending the functionality of REST proxy.