Bump GCC version to 4.7

- Drop pre-4.7 options handling and bump requirements to 4.7.
- Add Ubuntu toolchain Raring PPA and install g++ and g++-4.7 from it.

According to travis, colobot fails to build on GCC < 4.7 since 195d6cded05f7ef5bde695ee047b341a0265eab3, with errors like:
```
…/src/app/system_linux.h:41:23: error: expected ‘;’ at end of member declaration
…/src/app/system_linux.h:41:25: error: ‘override’ does not name a type
```

This commit bumps the GCC requirement to >= 4.7 and makes travis install the a valid gcc version from an official PPA.