Use scalaVersion for scalap dependency version
Upgrade to Scala 2.11.0-M5
Upgrade to scala-xml 1.0-RC4
update shouldn't fail on conflicting binary versions

scalap depends on scala-compiler, which depends (for the scaladoc part)
on scala-xml and scala-parser-combinators

More precisely, scala-compiler_2.11.0-M5 depends on
                     scala-xml_2.11.0-M4 and
      scala-parser-combinators_2.11.0-M4,
so that we get a binary version incompatibility warning.

It isn't really a problem, but we should consider doing something about this.

My preference: modularize scaladoc.