Pin pylint to old version

Pin pylint to old version to prevent new errors from appearing.

This is so that we don't have to refactor the codebase of a
bugfix branch, just to make pylint errors go away.

Author: jmoldow

requirements-dev.txt

@@ -3,10 +3,15 @@ bottle
 jsonpatch
 mock>=2.0.0
 pep8
-pylint
+
+# Pin to an old version so that new errors don't start appearing on the
+# bugfix-only 1.5 branch. Don't propagate this pinned version to the master
+# branch.
+pylint==1.5.5
+
 pytest>=2.8.3
 pytest-cov
 pytest-xdist
 sphinx>=1.5,<1.6
 sqlalchemy
-tox
+tox