Stop pinning pylint

We are many deprecation warnings in the travis logs for things that are fixed in newer versions of `pylint` or `pylint`'s dependencies.

So opening a PR to see if `master` passes on whatever version travis pulls in. Note that `pylint` >= 2.0 does not support python 2, so this will result in different versions of pylint running for python 2 vs python 3. Personally, I am just fine with this.

tox.ini

@@ -11,7 +11,7 @@ log_format = %(created)f %(filename)-23s %(threadName)s %(message)s
 deps =
     pytest
     pytest-cov
-    py{27,34,35,36,py}: pylint==1.8.2
+    py{27,34,35,36,py}: pylint
     py{27,34,35,36,py}: pytest-pylint
     pytest-mock
     mock