Use python3.6 for tests.

.travis.yml

@@ -1,5 +1,5 @@
 sudo: false
-language: python
+language: python3.6
 env:
   matrix:
     - ENV=test
@@ -11,7 +11,7 @@ matrix:
 install:
   - |
     if [ "$ENV" = "test" ]; then
-      pip install pytest
+      python3.6 -m pip install pytest
     fi
 script:
   - vim --version