This commit adds a build step to travis to auto-delete unstable archives older than
14 days (our regular release schedule) from Azure via ci.go purge.
The commit also pulls in the latest Azure storage code, also switching over from
the old import path (github.com/Azure/azure-sdk-for-go) to the new split one
(github.com/Azure/azure-storage-go).
I didn't add this initially because the command I tried was too slow.
The 'git for-each-ref ...' invocation takes 40ms on my machine. That
ought to be acceptable.
This commit tweaks the debian packaging tool:
* All build environment metadata can now be overriden on the command
line. This allows testing the CI build behaviour locally.
* -unstable packages now actually contain the binaries (oops)
* packages use Go 1.7 to build
* archiving is skipped for PR builds
