Install git version of Theano for python 3

The current release of Theano on PyPI only supports python 2. For python
3, Theano now is installed from the master branch of the Theano git
repo.

Using '<=0.6.1dev' as the required version is a hack to get pip to
install the git version of Theano. If set to '==0.6.1dev' pip will
download the git version but then throw an error because the git version
is still 0.6.0rc3. If set to '==0.6.0rc3', pip will give preference to
the pypi version.

requirements.txt is no longer coupled to setup.py. `pip install -r
requirements.txt` will now install the developmental version of Theano
for python 2 and 3.

Fixes #388