Keys in RevolverPlugin should not be in Global

When using Revolver with an sbt build having multiple projects, the
reLogTag will be set to the same value for each project, rather than
each projects' name.

This change removes the "in Global" qualifier from the keys in
RevolverPlugin

Consider the following build:

object MyBuild extends Build {
  lazy val foo =
    Project(id = "foo", base = file("foo")).settings(Revolver.settings: _*)

  lazy val bar =
    Project(id = "bar", base = file("bar")).settings(Revolver.settings: _*)
}

Without this fix, all "reLogTag" for all projects will be the same:
> $ sbt
Detected sbt version 0.13.0
Starting sbt: invoke with -help for other options
[info] Loading project definition from /home/jared/testsbt/project
[info] Set current project to testsbt (in build file:/home/jared/testsbt/)
> show reLogTag
[info] bar/*:reLogTag
[info] 	foo
[info] foo/*:reLogTag
[info] 	foo
[info] testsbt/*:reLogTag
[info] 	foo
>