Consider dropping pypy requirement

[Antar1011]

pypy can provide amazing speedups compared to regular python. On the other hand, it's incompatible with a lot of C-based packages (e.g. py_mini_racer, ujson and numpy/scipy/pandas (though hopefully I won't need any of those..)

So the question is: how big a bump can I get in performance from using pypy? The current scripts see limited gain. Is python execution even going to be the bottleneck? Will it actually run faster using pypy? Can I achieve similar gains using numba?

I should consider dropping the pypy req for now and then revisiting it after the system once the system in in an MVP state, where I can do profiling to answer these questions.

[Antar1011]

PyPy compatibility will be considered for the 0.8 release