Pin Ruby at 1.9.3

It's close to what the Bamboo stack uses (1.9.2), so when upgrading to
Cedar, we should continue to use it to mitigate upgrade difficulties.

It's not 1.9.2 (exactly what Bamboo uses) because 1.9.2 doesn't compile
on my dev machine.

Change-Id: Id93188cef83ac57d10892924baba87ce293d4468
Reviewed-on: http://gerrit.causes.com/38020
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>