Update Travis CI config

- Use Xenial for all tests
- Drop support for Python 3.2 and Python 3.3

.travis.yml

@@ -1,26 +1,16 @@
+dist: xenial
 language: python
+python:
+  - "2.7"
+  - "pypy"
+  - "3.4"
+  - "3.5"
+  - "3.6"
+  - "3.7"
+  - "nightly"
+  - "pypy3"
 
 sudo: false
 
-.mixins:
-  - &xenial
-    dist: xenial
-
-jobs:
-  include:
-    - python: "2.7"
-    - python: "pypy"
-    - python: "3.2"
-    - python: "3.3"
-    - python: "3.4"
-    - python: "3.5"
-    - python: "3.6"
-    - <<: *xenial
-      python: "3.7"
-    - <<: *xenial
-      python: "3.8-dev"
-    - python: "nightly"
-    - python: "pypy3"
-
 script:
   - ./test/test_travis