Fix for PyExecJS >= 1.0.5

Test plan:
% EXECJS_RUNTIME=Node python ./react/test/__main__.py
% EXECJS_RUNTIME=PhantomJS python ./react/test/__main__.py

It fails with JavaScriptCore runtime but does so in an older version (probably
because of Browserify).
Merge pull request #17 from andreypopp/pin-pyexecjs

Fix for PyExecJS >= 1.0.5