Use gethash, not getenv, to access config.

Fixes #393.
Do not throw an error if Flymake is not loaded.

Users not activating the flymake module could experience errors
when flymake was not loaded. Prevent these errors.

Fixes #394.
elpy-use-cpython: Set interpreter to correct value.

This now uses the argument string in Emacs 24.4, too, instead of
hardcoding it to "python".

Fixes #392.
fix boolean sort in ascending order
Merge pull request #402 from fleimgruber/master

fix boolean sort in ascending order
Add 24.4 to test runs.
Remove redundant call to (elpy-config--get-config)
Update README to mention jedi as an alternative to rope.
Add CONTRIBUTING.rst and clean up requirements*.txt
Make tests pass when not running in a virtualenv

The keys "virtual_env" and "virtual_env_short" are always expected
in the elpy config hash.  Add them (as nil) if VIRTUAL_ENV is not
defined in the environment.
Add 24.4 to Makefile, but remove from Travis.

Apparently, evm does not have 24.4 binary packages yet.
Re-add emacs 24.4, as it is now available on EVM.