Feature travis coverage

I feel a bit bad about all the stupid commits but it had to be pushed to test (having a local Travis would help).
CXX_FLAGS was not set properly which is very silly (but it was working for me).
A specific version of lcov needs to be used (installed from bin).
There is a dirty hack to make sure the proper version of gcov is used (same as gcc).
An additional program to display coverage results in the console is installed (gcovr).
The sed command might seem a bit scary but it just removes the right part of the display.