Unpin Cheetah dependency version

Fixes bug #1133216

We've been pinned to the same version of Cheetah since May 2011:

  Cheetah==2.4.4

this was released in December 2010 and there hasn't been a release since:

  https://pypi.python.org/pypi/Cheetah/2.4.4

There's no particular reason to believe a future release of Cheetah will break
us (only 6 commits since 2.4.4 and they all look like bugfixes), so we should
change to:

  Cheetah>=2.4.4

Change-Id: Id3b22b84d2d693776f89ccb1d0c99d4e42fcab62