Cleanup and tinkering.

* Now using main repo of `metric_fu` Gem, since @phantomwhale's changes were merged into what was released as 4.10.0;
* Removed DHH's "Welcome to Rails!" static index page;
* Added `pry` to `spec_helper.rb`.

RSpec: 4 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 9 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2