BF: fix travis version setting

Luckily Steven Silvester noticed that selecting Python versions for the
virtualenvs was not working : https://github.com/nipy/dipy/issues/487
Merge pull request #488 from matthew-brett/fix-travis-again

BF: fix travis version setting
TEST: compare against different energy profile with SSE2
Export build flags to __config__.py
TEST: use exported build flags to select oracle energy profile
TEST: skip regression tests when SSE2 is not supported
BF: apparently, 'has_key(...)' was removed from dict in python3
Generate the config file earlier
HACK: generate __config__ module as a cython extension to make sure it is generated before it is moved to the module install directory
Remove __config__.pyx, will be auto-generated