pycrypto _fastmath sometimes fails to build under PyPy

[pjenvey]

This only happens in environments where libgmp-dev or libmpir-dev are installed. Without those, _fastmath is disabled so the build succeeds.

It can be disabled via "with_gmp=no pip install pycrypto". We should be able to work that env variable into the requirements files.

The pypy docker build doesn't have those libs, so it builds fine

(pycrypto/pycrypto#59 for more information)

[pjenvey]

We actually can't workaround this in requirements files like I thought.

We could add a mention of the issue in the install docs

[pjenvey]

We'll fix this in the Makefile since the install docs direct you to run make to build autopush. That probably won't cover every potential case of this in the right environment, but it's just not that important (and eventually #785 will get rid of the pycrypto dep anyway)

[pjenvey]

and in tox.. that should cover it, really