Allow update to jline2.12

Hi, 

this is a follow-up to https://github.com/groovy/groovy-core/pull/507.

so creating just one Terminal seems to fix the bash problem for me. The other workaround proposed at https://github.com/jline/jline2/issues/163#issuecomment-54092550 would also work, but I feel the approach in this PR is cleaner anyway. This PR also includes a revert of the revert of adding jline 2.12. So I have no other problems with jline 2.12. But if you do not wish that migration, stillconsider cherry-picking the first commit to be future-safe.