fix RcovHotspot map bug & update rcov config docs

i was having a heck of time getting simple-cov (using the rcov formatter) integrated with metric_fu.  i tracked down two issues:

- the config instructions in the readme file were missing a step
- the rcov hotspot analyzer had a bug related to mapping the results

I added a spec for the MetricFu::RcovHotspot class.  It includes two examples for map.

there are 4 failing examples in rcov_spec.  it appears that these are failing in master: https://travis-ci.org/metricfu/metric_fu/jobs/11822975.  i didn't try a fix because i think you have a few pull requests trying to fix this.

I ran this against 2.0.0-p247.  I will keep an eye on travis to see if it passes the other target runtimes.

Let me know if you want any additional changes.