Fix missing quote in .travis.yml

.travis.yml

@@ -2,18 +2,13 @@ language: python
 sudo: required
 
 python:
- - "2.7
- - "3.6"
+- "2.7"
+- "3.6"
 
 matrix:
   include:
     - dist: trusty
       env: dist="14.04 LTS trusty"
-#    - dist: xenial
-#      env: dist="16.04 LTS xenial"
-# Travis-CI only proposes 14.04 LTS Trusty and there is no plan to update to 16.04 xenial
-# (c.f. https://github.com/travis-ci/travis-ci/issues/5821)
-# It is useless to add a second job because it will run in the same Ubuntu version (14.04)
 
 before_install:
 - sudo apt-get update -qq
@@ -32,4 +27,5 @@ script:
 notifications:
   email:
     on_success: never
-# More details: https://docs.travis-ci.com/user/notifications#Configuring-email-notifications
+
+# More details: https://docs.travis-ci.com/user/notifications