TST: flake8 should run on all python builds

[parthea]

From PR #39 ,
I received the following warning when running flake8 on branch tswast:gbq-37 but Travis did not report the warning.

Local run:

tony@tonypc:~/pydata-pandas-gbq$ flake8 pandas_gbq 
pandas_gbq/tests/test_gbq.py:136:61: F841 local variable 'e' is assigned to but never used

On Travis:
https://travis-ci.org/tswast/pandas-gbq/jobs/234914390

[parthea]

Flake8 is currently only running on python 3.6 in the Travis environment, whereas my local environment is python 2.7.

[jreback]

just turn it on additionally for 2.7