Drop support for Go 1.8, 1.9, 1.10

[sbuss]

Primarily, this is deleting the travis config for those versions and cleaning up any files with a build tag for these versions.

[roffjulie]

Update: https://github.com/golang/appengine/pull/217 attempted to do this, but this broke some downstream testing and so we're going to hold off on that deletion for a few weeks and then try again.