Make plugin source compatible with SBT 0.13.

     ~/code/sbt-xjc sbt
    Detected sbt version 0.12.4
    Starting sbt: invoke with -help for other options
    Using /Users/jason/.sbt/0.12.4 as sbt dir, -sbt-dir to override.
    [info] Loading project definition from /Users/jason/code/sbt-xjc/project
    [info] Set current project to sbt-xjc (in build file:/Users/jason/code/sbt-xjc/)
    > set scalaVersion in Global := "2.10.2"
    [info] Defining */*:scala-version
    [info] The new value will be used by */*:cross-scala-versions, */*:scala-binary-version and 10 others.
    [info]  Run `last` for details.
    [info] Reapplying settings...
    [info] Set current project to sbt-xjc (in build file:/Users/jason/code/sbt-xjc/)
    > set sbtVersion in Global := "0.13.0-RC2"
    [info] Defining */*:sbt-version
    [info] The new value will be used by */*:sbt-binary-version, */*:sbt-dependency and 1 others.
    [info]  Run `last` for details.
    [info] Reapplying settings...
    [info] Set current project to sbt-xjc (in build file:/Users/jason/code/sbt-xjc/)
    > scripted
    ...
    Running sbt-xjc / compile-and-test
    Running sbt-xjc / fluent
    Running sbt-xjc / simple
    [success] Total time: 102 s, completed Aug 1, 2013 9:53:45 AM