CI(Travis): Set default OS to `linux`, dist to `xenial`

Default dist is `xenial` anyway.

diff --git a/.travis.yml b/.travis.yml
index 31246e39cb8c83c5e7fcbc5b0fea7fcb417884e0..e2280bf511b1f05339d1a03c8f26400612a8383b 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,7 @@
+os: linux
+
+dist: xenial
+
 language: python
 
 python:
@@ -16,12 +20,10 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37
   - python: "2.7"
     env: TOXENV=py27-flake8
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37-flake8
 
 install: