Updare README about 0.5.1

[ngocdaothanh]

I see that v0.5.1 has been released:
https://github.com/backchatio/xsbt-scalate-generate/releases

But README is still saying:
addSbtPlugin("com.mojolly.scalate" % "xsbt-scalate-generator" % "0.5.0")

One more thing, when I change 0.5.0 above to 0.5.1, SBT can't find the new version:
[error] (*:update) sbt.ResolveException: unresolved dependency: com.mojolly.scalate#xsbt-scalate-generator;0.5.1: not found

[fizbin]

@casualjim Are you still actively maintaining this project? Why was the 0.5.1 release never pushed to where maven central et al. could find it?

[casualjim]

I no longer am, but this is the same code only with an active maintainer. https://github.com/skinny-framework/sbt-scalate-precompiler