Fix SBT dependencies

With double '%' SBT tries to find the dependencies compiled with the
locally used Scala version. Since Joda doesn't use SBT/Scala this
results in an error like:

[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  ::          UNRESOLVED DEPENDENCIES         ::
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  :: joda-time#joda-time_2.10;2.3: not found
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::

See:
http://www.scala-sbt.org/release/docs/Library-Management.html#Dependencies