[Fix] Ensure external coverage file exists before running coverage metrics

RCov will now enable itself if it finds the coverage/rcov/rcov.txt artifact the simplecov rcov text formatter creates.

This also fixes #78 

This is how [metric_fu runs both the html and rcov text formatters at the same time](https://github.com/metricfu/metric_fu/blob/d372d0001db1efb2ecd39368d211c784a8e01b85/spec/spec_helper.rb#L4)

```ruby
require 'rspec/autorun'

if ENV['COVERAGE']
  require 'simplecov'
  formatters = [SimpleCov::Formatter::HTMLFormatter]
  begin
    puts '[COVERAGE] Running with SimpleCov HTML Formatter'
    require 'simplecov-rcov-text'
    formatters << SimpleCov::Formatter::RcovTextFormatter
    puts '[COVERAGE] Running with SimpleCov Rcov Formatter'
  rescue LoadError
    puts '[COVERAGE] SimpleCov Rcov formatter could not be loaded'
  end
  SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[ *formatters ]
  SimpleCov.start
end
```