Re-enable leak checking in address sanitizer builds

[springmeyer]

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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 https://github.com/mapnik/node-mapnik/commit/cd5f6b419ca447840b67cf26c10e050de260b91b
• [ ] Evaluate if any of the remaining leaks are serious and need fixing

[springmeyer]

For reference, here are the errors coming from the leak sanitizer (from a branch that I re-enabled it on): https://travis-ci.org/mapnik/node-mapnik/jobs/219065861