templatetags: Drop support for non-diffmatchpatch highlight_diffs()

Also drop reliance on ttk diff-match-patch, use pypi instead.
docs: older pip might install pre-releases

Looks like pip might install e.g. 2.5.1-rc1 when 2.5.1 has been
released.  It would seem that their version detective work isn't working
in those cases.

This occured on Debian Wheezy (7.4) and pip 1.1

I don't think we want to remove the old versions from pip because we
want to allow fallback path for users if required and because these are
FOSS releases so we shouldn't just remove them.

So make a note in the install guide for this possible occurance.

Fixes bug 3185 to some extent.
docs: make docs-review launches local docs in browser

A quick way to launch the docs in a browser for easy review.
req: Hide min-requirements.txt

Causes too much confusion.  We can get it via make if we need it for
testing.
req: New minimum build version

Tested the doc build and fails with 1.1 but works on 1.2