MB-12913: Use C++11 compiler (gcc 4.8) for CentOS 6 jenkins docker image

Update CentOS 6 image to use GCC 4.8 from CentOS' port of RHEL's
devtools repository.

Change-Id: I84e80ac4c6cf278126c8f436dbf63911c6c35abd
Reviewed-on: http://review.couchbase.org/44465
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>