Update travis config to specify Python 3.6

Attempt to fix Travis error:
https://travis-ci.com/adafruit/adafruit-beaglebone-io-python/builds/140636005

```
1.74s$ git clone --depth=50 --branch=issue308 https://github.com/adafruit/adafruit-beaglebone-io-python.git adafruit/adafruit-beaglebone-io-python
0.01s0.01s$ source ~/virtualenv/python3.6/bin/activate
$ python --version
Python 3.6.7
$ pip --version
pip 19.0.3 from /home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/pip (python 3.6)
install
1.54s$ pip install tox
5.96s$ tox
GLOB sdist-make: /home/travis/build/adafruit/adafruit-beaglebone-io-python/setup.py
py27 create: /home/travis/build/adafruit/adafruit-beaglebone-io-python/.tox/py27
py27 inst: /home/travis/build/adafruit/adafruit-beaglebone-io-python/.tox/.tmp/package/1/Adafruit_BBIO-1.1.1.zip
py27 installed: Adafruit-BBIO==1.1.1
py27 run-test-pre: PYTHONHASHSEED='645938357'
py27 run-test: commands[0] | echo 'run pytest on beaglebone'
run pytest on beaglebone
py34 create: /home/travis/build/adafruit/adafruit-beaglebone-io-python/.tox/py34
ERROR: InterpreterNotFound: python3.4
___________________________________ summary ____________________________________
  py27: commands succeeded
ERROR:  py34: InterpreterNotFound: python3.4
The command "tox" exited with 1.
Done. Your build exited with 1.
```

Author: pdp7

.travis.yml

@@ -1,4 +1,6 @@
 language: python
+python:
+    - "3.6"
 install:
   - pip install tox 
 script: