Require enum34 in Python 2 only

The enum34 package is included in the standard library since Python 3.4, so there's no need to add it as a requirement when installing PyRDF in a Python 3.4+ environment.
In fact, this requirement is messing with pip (e.g. pypa/pip#8214)

requirements.txt

@@ -1,2 +1,2 @@
-enum34
+enum34 ; python_version < '3.4'  # Requires pip 6.0 and later
 nose