Drop python 3.4 and 3.5 from travis. (#712)

Add 3.7 and 3.8
diff --git a/.travis.yml b/.travis.yml
index d32d411..fdeb58d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,12 +1,8 @@
 language: python
 python:
-  - "3.4"
-  - "3.5"
   - "3.6"
-matrix:
-  include:
-    - python: "3.7"
-      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+  - "3.7"
+  - "3.8"
 install:
   python setup.py install
 script: