Support sbt 0.13

Hey Tim,

I know you've not touched this plugin in years, but I hope you can find time to check out this PR and publish `0.4.2` for both sbt `0.12` and `0.13`.  It's set up such that `sbt "+ publish"` will do it.  I've tested that it works well with `0.13` and `xsbt-web-plugin 0.9.0`.  Let me know if there is anything else I can do to help you dust this one off and update it.

Joe