Change master's version to 0.7.0-SNAPSHOT

Having created https://github.com/brooklyncentral/brooklyn/tree/0.6.x, and from that created https://github.com/brooklyncentral/brooklyn/tree/0.6.0-rc.2 - it means we can now change master to be 0.7.0-SNAPSHOT.

Note the deliberate naming of "0.6.x" (in contrast to the previous 0.4 and 0.5). This is because the ".x" suffix makes it very clear it is not a single version, but instead the snapshot branch from which all 0.6.1 etc branches will be taken.