Updated RSpec coverage environment variable

The original `NO_COVERAGE` environment variable was awkward to read due to the double negative so `COVERAGE` is being used with an explicit "no" to disable. A message is printed to the console, when disabled, so there are no surprises.

Milestone: minor

spec/spec_helper.rb

@@ -2,7 +2,9 @@
 
 require "simplecov"
 
-unless ENV["NO_COVERAGE"]
+if ENV["COVERAGE"] == "no"
+  puts "SimpleCov skipped due to being disabled."
+else
   SimpleCov.start do
     add_filter %r(^/spec/)
     enable_coverage :branch