Add default credentials definition in build.sbt.

Credentials file is configured according to this sbt documentation page:
http://www.scala-sbt.org/release/docs/Detailed-Topics/Publishing.html#credentials
Release scala-parser-combinators 1.0.0-RC4.
Switch back to SNAPSHOT version after 1.0.0-RC4 release.
Merge remote-tracking branch 'scala/master'