Use SimpleCov.add_filter block instead of string

Using a string is almost always a bad idea, because if any part of
the path contains the string, it will filter out the path. Switch
to using a block and doing a match on the entire path prefix.

Author: jeremyevans

spec/spec_helper.rb

@@ -2,7 +2,7 @@
   require 'simplecov'
   SimpleCov.start do
     enable_coverage :branch
-    add_filter "/spec/"
+    add_filter{|f| f.filename.match(%r{\A#{Regexp.escape(__dir__)}/})}
     add_group('Missing'){|src| src.covered_percent < 100}
     add_group('Covered'){|src| src.covered_percent == 100}
   end