gdb: move back to 7.7 until issues with 7.8 are understood

The following tests hangs in CMSSW "TestFWCoreServicesDriver"

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #1014 from davidlt/gdb-back-to-7.7

gdb: move back to 7.7 until issues with 7.8 are understood