Use less convoluted way to use Python 3.7 in CI

This seems to be all right now, no need for distro switch

Author: jstasiak

.travis.yml

@@ -4,14 +4,13 @@ cache: pip
 python:
   - "3.5"
   - "3.6"
+  - "3.7"
   - "nightly"
   - "pypy3.5"
   - "pypy3"
 matrix:
   allow_failures:
     - python: "nightly"
-  include:
-    - { python: "3.7", dist: xenial, sudo: true }
 install:
   - pip install --upgrade coveralls pytest "pytest-cov>=2.5.1" dataclasses
   # mypy can't be installed on pypy