Bump version number to 3.0.0-SNAPSHOT.

Once we bump the version, I can branch for release/3.0.x.

I removed some versioned dependencies between our own core plugins (sdt.debug
to sdt.core, sdt.spy to sdt.core), as I think they are not needed and are just
a pain to maintain.
Merge pull request #295 from dragos/set-versions

Bump version number to 3.0.0-SNAPSHOT.