[Feature] Run external metrics without shelling out

- Metrics are now run via the GemRun class via the Open3 popen call
- Metric versions are determined by the GemVersion class that uses RubyGems to read in the gem specification and find or build dependencies.  (metric_fu_requires is also removed)
- `--debug-info` now returns gem dependencies
- MetricFu still installs some executables such as mf-reek, mf-churn, but they are no longer used by the library.  The run via GemRun, as well.  I'd include a deprecation warning for using them if we had a deprecation API :)  They're main purpose was to ensure that the expected versions of libraries were run when shelling out.  We now manage that within metric fu by GemRun and GemVersion

This has been testing on cruby 1.9, 2.0, and 1.9 mode for jruby and rbx, but not on the windows platform.  It should work :)