update from googleapis (googleapis#85)

* disable go1.6

gRPC imports golang.org/x/sys/unix which currently does not build on
go1.6. Depending on how compatibility story plays out, we'll either drop
1.6 support or add back 1.6 CI.

This commit also slightly changes the CI version "structure".
We previously list all Go versions to test against. This commit
makes us test against 1.7 and the latest version.
This means we don't have to manually bump version when new releases come
out. We have to be doing something really strange if we build against
1.7 and 1.10, but not 1.8 and 1.9.