Include Python 3.7 and 3.8 on TravisCI

Author: peterjc

.travis.yml

@@ -15,6 +15,10 @@ jobs:
       python: "3.5"
     - stage: test
       python: "3.6"
+    - stage: test
+      python: "3.7"
+    - stage: test
+      python: "3.8"
 
 before_install:
  - "if grep '\t' *.py *.rst; then echo 'Tabs are bad, please use four spaces.'; false; fi"