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

Default dist is `xenial` anyway.

diff --git a/.travis.yml b/.travis.yml
index ddc93951d97f11da52a4fa723c49c54a4125ad37..a6df1127a340eec0771f34dfa1e26a6aa39aefec 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,7 @@
+os: linux
+
+dist: xenial
+
 language: python
 
 python:
@@ -23,7 +27,6 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37
   - python: "2.7"
     env: TOXENV=py27-postgres
@@ -34,7 +37,6 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36-postgres
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37-postgres
   - python: "2.7"
     env: TOXENV=py27-sqlite
@@ -45,12 +47,10 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36-sqlite
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37-sqlite
   - python: "2.7"
     env: TOXENV=py27-flake8
   - python: "3.7"
-    dist: xenial
     env: TOXENV=py37-flake8
 
 install: