bump core version to pre7
bump core version to pre7

Terracotta-OSS/terracotta-platform,chrisdennis/terracotta-platform,albinsuresh/terracotta-platform,lorban/terracotta-platform,AbfrmBlr/terracotta-platform,albinsuresh/terracotta-platform,Terracotta-OSS/terracotta-platform,chrisdennis/terracotta-platform