Require minimum gcc version to be 4.8.

This version supports all of the used c++11 features.
Install GCC 4.8.1 on travis.

Instructions taken from http://askubuntu.com/questions/271388/how-to-install-gcc-4-8
Merge pull request #464 from endJunction/SetMinimumGCCVersion481

Require minimum gcc version to be 4.8.1.