require specifying the version for gh-pages

From now on, we are only maintaining one build of the IPython docs per major release, e.g. latest 1.x docs served at ipython-doc/1/. This makes gh-pages no longer guess based on git tag. To build the 1.x docs:

    VERSION=1 make gh-pages
Merge pull request #5108 from minrk/gh-pages-major

require specifying the version for gh-pages