travis: use different dists/sudo for different python versions

.travis.yml

@@ -1,8 +1,3 @@
-# we don't actually need sudo, but that kicks us onto GCE, which means
-# "group: edge" will let us get Ubuntu 14.04 LTS (xenial)
-sudo: true
-group: edge
-dist: xenial
 language: python
 cache: pip
 before_cache:
@@ -10,14 +5,6 @@ before_cache:
 branches:
   except:
     - /^WIP-.*$/
-python:
-  - "2.7"
-  - "3.3"
-  - "3.4"
-  - "3.5"
-  - "3.6"
-  - "3.7"
-  - "nightly"
 install:
   - pip install -U pip tox virtualenv codecov
 before_script:
@@ -30,6 +17,31 @@ script:
 after_success:
   - codecov
 matrix:
+  include:
+    - python: 2.7
+      dist: trusty
+      sudo: false
+    - python: 3.3
+      dist: trusty
+      sudo: false
+    - python: 3.4
+      dist: trusty
+      sudo: false
+    - python: 3.5
+      dist: trusty
+      sudo: false
+    - python: 3.6
+      dist: trusty
+      sudo: false
+    - python: 3.7
+      dist: xenial
+      # we don't actually need sudo, but that kicks us onto GCE, which means
+      # "group: edge" will let us get Ubuntu 14.04 LTS (xenial)
+      #group: edge
+      sudo: true
+    - python: nightly
+      dist: trusty
+      sudo: false
   allow_failures:
     - python: "3.3"
     - python: "nightly"