Move completely to go15

[christiangriset]

For context, see https://clever.atlassian.net/browse/INFRA-1112

This PR was autogenerated. @jefff you were chosen as the most active commiter of this repo, please review and check that builds still pass.
Some known, but difficult to automate, errors are:

• In the Makefile and test_drone.sh, performing go get on the repo while it's now only necessary for packages such as errcheck or lint.
• Makefile testing vendored dependencies (which in turn tries to compile them, oftentimes creating import errors).
• Dependencies aren't compatible with go1.5. For example appasaurus ran into this with gopkg.in/mgo.v2 (the solution here is to run go get -u gopkg.in/mgo.v2).

If there are any problems, feel free to ping @cgclever for troubleshooting.

For understanding Godeps and vendoring see this doc.

Thank you!

[kvigen]

@cgclever i fixed a bug due to a backwards incompatible change between go 1.2 and 1.5. Can you review it https://github.com/Clever/oplog-replay/commit/6388e03ffe82bd2d2b81493c5d27cb27a45df426. If you're good with that, then this lgtm

[christiangriset]

Interesting gotcha, very subtle. Thanks for catching it!