Correct calls to MetricFu.logger.mf_debug to MetricFu.logger.debug

Fixes https://travis-ci.org/metricfu/metric_fu/jobs/23891204

NoMethodError: private method `mf_debug' called for <MetricFu::Logger:0x1665e34c>
Check if metric is activated; was bypassing ripper check
Adding launchy gem as runtime dependency
Using launchy to display output file instead of system open
Now allowing file to be opened on any platform Launchy supports
Making sure URIs are escaped to handle ugly paths