In #3262 the minimum version of freetype was lowered to 2.3, but it looks like this change wasn't propagated to the 1.4.x branch. CentOS 6 ships 2.3.11.

I just installed it against my local setup after lowering the required version and all the tests seem to pass against 2.7.8 and 3.4.1. I'd be happy to test anything you need to lower the version back to 2.3.

Incidentally, the currently documented freetype minimum claims to be 1.4, see http://matplotlib.org/users/installing.html#optional-dependencies.