frontier_client: bump version to 2.8.8

Fix issue where frontier_client terminates with 1 (non-zero)
return code on success. It's a mininmal change comparing to
2.8.7.

Discussion:
  https://hypernews.cern.ch/HyperNews/CMS/get/sw-develtools/1876.html

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #115 from davidlt/IB/CMSSW_7_0_X/stable

[CMSSW_7_0_X] frontier_client 2.8.8