Test on 3.7-dev

Author: konstin

.travis.yml

@@ -1,9 +1,14 @@
 sudo: required
 language: python
-python:
-  - "2.7"
-  - "3.5"
-  - "3.6"
+
+matrix:
+  include:
+  - python: "2.7"
+  - python: "3.5"
+  - python: "3.6"
+  - python: "3.7-dev"
+  allow_failures:
+  - python: "3.7-dev"
 
 env:
   - RUST_VERSION=nightly