Tests on Python 3 are slow

[jhamrick]

From @takluyver:

Travis tests on Python 3 are really sloooow, because the numpy in the Ubuntu repositories is incompatible with the requirements spec, so it has to compile numpy and pandas from scratch, which is slow. If we relaxed the requirements a bit, we could apt-get install pandas. The Ubuntu 12.04 repositories have pandas 0.10.1, and we could add the NeuroDebian repo, which has 0.12.