fix travis error Unable to download 3.7 archive.

Author: ht-albert

.travis.yml

@@ -12,6 +12,9 @@ matrix:
     python: 3.6
   - os: linux
     python: 3.7
+    dist: xenial
+    sudo: true
+
 install:
 - pip install -r requirements/test.txt
 - pip install -r requirements/sdk.txt