Combine the pep8 and check_isort builds into one

there's really no point spinning up two separate jobs for these.

.travis.yml

@@ -22,7 +22,7 @@ matrix:
     env: TOX_ENV=packaging
 
   - python: 3.6
-    env: TOX_ENV=pep8
+    env: TOX_ENV="pep8, check_isort"
 
   - python: 2.7
     env: TOX_ENV=py27
@@ -46,9 +46,6 @@ matrix:
     services:
       - postgresql
 
-  - python: 3.6
-    env: TOX_ENV=check_isort
-
   - # we only need to check for the newsfragment if it's a PR build
     if: type = pull_request
     python: 3.6