Add __ne__ method to Requirement class

It seems like if it has an `__eq__` method, it should probably have a `__ne__` method.

I ran into this while working on pip -- see https://github.com/pypa/pip/pull/2493#discussion_r25955295

pkg_resources/__init__.py

@@ -2907,6 +2907,9 @@ class Requirement:
             self.hashCmp == other.hashCmp
         )
 
+    def __ne__(self, other):
+        return not self == other
+
     def __contains__(self, item):
         if isinstance(item, Distribution):
             if item.key != self.key: