Drop py3.4 build, add py3.7 build

[benbariteau]

Python 3.4 is EOL and py3.7 has a good bit of life ahead of it, while not being too new.

[coveralls]

Coverage remained the same at 98.658% when pulling 2f4d46d on firba1:drop-py3.4-add-py3.7 into e683991 on Yelp:master.

[danielpops]

@firba1 do you also want to update https://github.com/Yelp/fido/blob/master/.travis.yml#L5 to remove versions we don't want to run in travis?

[benbariteau]

@firba1 do you also want to update https://github.com/Yelp/fido/blob/master/.travis.yml#L5 to remove versions we don't want to run in travis?

Since this is mostly a fix for the internal build, let's do one thing at a time :P