travis: remove python3.4, add python3.7

.travis.yml

@@ -15,14 +15,18 @@ matrix:
   include:
     - python: 2.7
       env: TOXENV=py27-cov
-    - python: 3.4
-      env: TOXENV=py34-cov
     - python: 3.5
       env: TOXENV=py35-cov
     - python: 3.6
       env:
         - TOXENV=py36-cov
         - BUILD_DIST=true
+    - python: 3.7
+      env: TOXENV=py37-cov
+      # required to run python3.7 on Travis CI
+      # https://github.com/travis-ci/travis-ci/issues/9815
+      dist: xenial
+      sudo: true
     - python: pypy2.7-5.8.0
       # disable coverage.py on pypy because of performance problems
       env: TOXENV=pypy-nocov