Update .travis.yml to use python 3.7 (python#850)

because the master branch is 3.7 so it's only logical to have py 3.7 as well

Author: endormi

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 dist: xenial
-python: 3.6
+python: 3.7
 before_install:
   - sudo apt-get update
   - sudo apt-get install -y hunspell hunspell-fr-comprehensive