Use pip --user instead of --install-option

I don't know if there is a specific reason that --install-option='--prefix=~/.local' is used instead of pip's --user, but it seems to break things when installing Theano without root access (both myself and Jae Hyun had this issue).
Removed installation instructions for python < 2.6
Merge pull request #1758 from bartvm/patch-1

Use pip --user instead of --install-option