only xenial for 3.7 (#160)

Author: juancarlostong

Diff for: .travis.yml

@@ -4,7 +4,7 @@ python:
   - "3.4"
   - "3.5.5"
   - "3.6"
-  - "3.7"
+# - "3.7"  is handled in 'Test' job using xenial as Python 3.7 is not available for trusty.
   - "pypy"
   - "pypy3"
 install: "pip install -r requirements/core.txt;pip install -r requirements/test.txt"
@@ -33,3 +33,7 @@ jobs:
       script:
         - "ci/trigger_fullstack-sdk-compat.sh"
       after_success: skip
+    - stage: 'Test'
+      # python 3.7 not available for trusty
+      dist: xenial
+      python: "3.7"