Bump sbt 0.13.1 to 0.13.7

- sbt 0.13.7
- Fix deprecation warnings (method x in class PathFinder is deprecated: Use pair.)

```
    [info] Compiling 5 Scala sources to /scalatra-sbt/target/scala-2.10/sbt-0.13/classes...
    [warn] /scalatra-sbt/src/main/scala/org/scalatra/sbt/DistPlugin.scala:28: method x in class PathFinder is deprecated: Use pair.
    [warn]       val jars = libs.descendantsExcept(GlobFilter("*"), excl) x flat(tgt / "lib")
    [warn]                                                                ^
    [warn] /scalatra-sbt/src/main/scala/org/scalatra/sbt/DistPlugin.scala:31: method x in class PathFinder is deprecated: Use pair.
    [warn]         files x rebase(dir, tgt / "lib")
    [warn]               ^
    [warn] /scalatra-sbt/src/main/scala/org/scalatra/sbt/DistPlugin.scala:74: method x in class PathFinder is deprecated: Use pair.
    [warn]         IO.copy(files x rebase(wr, tgt / "webapp"))
    [warn]                       ^
    [warn] /scalatra-sbt/src/main/scala/org/scalatra/sbt/DistPlugin.scala:83: method x in class PathFinder is deprecated: Use pair.
    [warn]       val paths = files x rebase(tgt, nm)
    [warn]                         ^
    [warn] four warnings found
```