Update travis build gcc to 4.9

I was reading your tests using gcc and the builds errored from `unrecognized -std=c++11 option` errors. Seems like your gcc 4.8 update doesn't work and the build still uses the preinstalled 4.6. 

I have hardcoded the correct update on the `.travis.yml` file and the tests are compiled successfully. Anyway, the `rxcppv2_test` test halts with no output after just calling it, and travis aborts build after 10 minutes of no response. I'm still working on it.

If everything works as expected, I will put the GCC4.9 install on your `travis-install.sh` instead of hardcoding it on the travis yaml.