Drop GCC 4.6 support

[piotrdz]

In my latest commits, I broke the GCC 4.6 build on Travis. The problem is that I've used non-static member initialization "class X { int x = 12; }" and the new C++11 using alias ("using Alias = Type;").

I find that both features are nice to have in our code and I feel that we should be gradually moving on to proper C++11 support, not to mention C++14. Right now we have to do weird things to get Colobot to compile under GCC 4.6, 4.7 and of course MSVC 2013 which is even more lacking in some ways when it comes to C++11.

I would therefore propose to drop GCC 4.6 which is now more than 4 years old. GCC 4.7 was released in 2012, and I think that pretty much any recent Linux distribution has at least GCC 4.7 so I don't think it will be a problem for any of our users.

[tomangelo]

Are there any arguments to keep support for GCC 4.6? There is GCC 4.7, 4.8 and 4.9, so I don't think that supporting 4.6 is really necessary.

[tomaszkax86]

We should definitely strive towards refactoring to proper C++11 code. Just make sure we can compile on some of the most recent versions of GCC and MSVC.

[piotrdz]

All right, since we all agreed on it, I've disabled GCC 4.6 support and the GCC build on Travis.