Require python-pygments 1.5 or later

Starting with version 1.5, python-pygments has the CFEngine3 parser
included.

Also the reference to pygments_cfengine_lexer.py in
http://zzamboni.org/blog/cfengine3-lexer-for-pygments is stale.
Merge pull request #45 from kkaempf/fix-python-pygments-reference

Require python-pygments 1.5 or later