Use xenial dist

Author: jkimbo

.travis.yml

@@ -1,16 +1,18 @@
 language: python
-sudo: false
+dist: xenial
+
 python:
-- 2.7
-- 3.3
-- 3.4
-- 3.5
-- pypy
+  - 2.7
+  - 3.3
+  - 3.4
+  - 3.5
+  - pypy3
 install:
-- pip install pytest pytest-cov coveralls flake8
-- pip install .
+  - pip install pytest pytest-cov flake8
+  - pip install .
 script:
-- py.test --cov=graphql_relay
+  - py.test --cov=graphql_relay
 # - flake8
 after_success:
-- coveralls
+  - pip install coveralls
+  - coveralls