increase minimum version of google-cloud-core to 1.6.0

[tswast]

Once enough time has passed to give people time to upgrade (TBD how long that is), we should increase the minimum version and clean up and logic that switches based on package version that was added in #492.

[tswast]

I did a bit more research on how the Python data science community supports core library versions. They have a rolling 2-year window. https://github.com/pydata/pandas-gbq/issues/357

I still want to do some analysis of how quickly people upgrade core libraries in practice to see if 2-years is the right window. (Hopefully people are upgrading much more frequently than this)