travis: drop 1.4 build, add 1.6 build
travis: drop 1.4 build, add 1.6 build

Change-Id: I4eb0347f3bebe86f04075986d6cd89fc62d5bb52
Reviewed-on: https://code-review.googlesource.com/4210
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go