[ci] allow lint failure on travis

isort changes its mind so often that this lint build is becoming more of a
nuisance than helpful

Author: jcb91

.travis.yml

@@ -82,6 +82,7 @@ matrix:
   allow_failures:
     - env: TOXENV=appveyorartifacts
     - env: TOXENV=docs_linkcheck
+    - env: TOXENV=lint
 env:
   global:
     - LD_PRELOAD=/lib/x86_64-linux-gnu/libSegFault.so