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

Default dist is `xenial` anyway.

diff --git a/.travis.yml b/.travis.yml
index fc8ae1f2b787cd844c441ebfbc24512eff4f9eb4..451b3018e9df1d3e93b3125e0b235c3577e8602e 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,7 @@
+os: linux
+
+dist: xenial
+
 language: python
 
 python:
@@ -16,7 +20,6 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37
   - python: "pypy"
     env: TOXENV=pypy