Update Python3 versions in .travis.yml (#57)

- Removes Python 3.4 as no longer supported
- Python 3.5, 3.6, 3.7, and 3.8 are the currently supported releases

Author: DavidMuller

.travis.yml

@@ -1,8 +1,10 @@
 language: python
 python:
   - "2.7"
-  - "3.4"
+  - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
 
 # command to install dependencies
 install: pip install -r requirements.txt