Fix travis config (#4)

* Use xenial in travis config

* Only have travis-ci run on Flask-supported versions + nightly

Author: red-coracle

.travis.yml

@@ -1,18 +1,19 @@
-sudo: false
+sudo: required
 language: python
+dist: xenial
 python:
   - 2.7
   - 3.4
   - 3.5
   - 3.6
   - 3.7
-  - "3.8-dev"
   - "nightly"
   - "pypy"
   - "pypy3"
 
 matrix:
   allow_failures:
+    - python: "nightly"
     - python: pypy
     - python: pypy3