python: fix python path setting in plain cmake workspaces

In plain cmake workspaces the python install dir is set to
lib/python2/dist-packages instead of lib/python2.7/dist-packages

I checked the catkin/cmake/python.cmake file and in that file there is
an extra check for the version, so I assume that we should only use
the major version in case the major version is 3 or higher.
Merge pull request #618 from smits/indigo-devel

python: fix python path setting in plain cmake workspaces