Migrate to sbt 0.13.2. This requires modifying the build file and the version
of the assembly plugin.

Note: Bluejelly still uses Scala 2.9.x to compile (see projects/Build.scala).
By default sbt would attempt compiling with 2.10.3, but compilation throws
a lot of warnings and (critically), the bjc lexer returns the error token
instead of EOI when consuming its input. No idea why yet :(