Re-enable leak checking in address sanitizer builds #747

Currently the leak sanitizer (part of address sanitizer) is reporting a number of leaks. They look maybe spurious to me? Maybe the would go away if mapnik itself were instrumented or if libc++ were instrumented as well? So the next actions here are to:

• Instrument mapnik/libc++
• Build node against instrumented libc++
• Re-enable the leak checking that was disabled in cd5f6b4
• Evaluate if any of the remaining leaks are serious and need fixing