Add a flake8 job to Travis CI

Also switch to xenial by default and upgrade PyPy versions.

And reindent the YAML to use 2-space indents, as that's the style I've
become accustomed to from Ansible.

.travis.yml

@@ -1,23 +1,25 @@
 language: python
-sudo: false
+dist: xenial
 python:
-    - 2.7
-    - 3.4
-    - 3.5
-    - 3.6
-    - nightly
-    - pypy
-    - pypy3
+  - 2.7
+  - 3.4
+  - 3.5
+  - 3.6
+  - 3.7
+  - nightly
+  - pypy2.7-6.0.0
+  - pypy3.5-6.0.0
 matrix:
-    include:
-        - python: 3.7
-          dist: xenial
-          sudo: true
+  include:
+    - name: flake8
+      install: pip install flake8
+      script: flake8 *.py
+      after_success:
 install:
-    - pip install coverage coveralls
+  - pip install coverage coveralls
 script:
-    - coverage run --source=profilehooks test_profilehooks.py
+  - coverage run --source=profilehooks test_profilehooks.py
 after_success:
-    - coveralls
+  - coveralls
 notifications:
-    email: false
+  email: false