Travis: use Ubuntu Xenial (#302)

Closes #302

.travis.yml

@@ -4,21 +4,19 @@ python:
   - '3.4'
   - '3.5'
   - '3.6'
+  - '3.7'
+
+dist: xenial
 
 cache: pip
 
 jobs:
   include:
-    - stage: test
-      python: '3.7'
-      dist: xenial
     - python: '3.6'
       env: TOXENV=docs
     - python: '3.7'
-      dist: xenial
       env: TOXENV=djmaster
     - python: '3.7'
-      dist: xenial
       env: TOXENV=lint
 
     - stage: publish
@@ -65,7 +63,6 @@ after_success:
 branches:
   only:
     - master
-    - /^release\/.*$/
     - /^v?\d+\.\d+(\.\d+)?(-?\S+)?$/
 
 stages:
@@ -73,7 +70,6 @@ stages:
   - name: publish
     if: tag IS present
 
-
 notifications:
   email:
     on_success: always