CLN: Warn that default dialect is changing to "standard"

[tswast]

Towards issue #195

CC @RafaelAMello

[tswast]

Travis:

• https://travis-ci.org/tswast/pandas-gbq/builds/416461574 Not sure why 2.7 failed. It passes locally.
• https://travis-ci.org/tswast/pandas-gbq/builds/416476855

[max-sixty]

Thanks a lot for doing all that - sorry you had to go through all the updates. Looks great from here

[max-sixty]

I been using https://github.com/ambv/black on some small projects, which helps with the formatting for these sorts of broad changes (much easier than autopep8)

It's a big initial diff for existing projects, though

[tswast]

black looks interesting. pandas-gbq has some inconsistent formatting, so applying black might be a good idea anyway.