Use absolute URI in CODE_OF_CONDUCT

sphinx (at least the version used by readthedocs) appears to interpret <http:contributor-covenant.org>`__ as http://khmer.readthedocs.org/en/v1.2/dev/contributor-covenant.org.  Using an absolute URI (despite this format appearing in the restructuredText documentation) should fix this.

github appears to render it fine, however...
Merge pull request #684 from jsspencer/master

Use absolute URI in CODE_OF_CONDUCT