Add explicit 3.6 support

Author: ajrgrubbs

.travis.yml

@@ -2,6 +2,7 @@
 dist: xenial
 language: python
 python:
+  - "3.6"
   - "3.7"
 script:
   - python setup.py test

README.md

@@ -8,6 +8,10 @@ It is not the same as the Python Standard Library's struct (e.g., `import struct
 Read the proposal:<br/>
 https://github.com/ethereum/EIPs/blob/master/EIPS/eip-712.md
 
+#### Supported Python Versions
+- `3.6`
+- `3.7`
+
 ## Install
 ```bash
 pip install eip712-structs