Exclude /spec/ from code coverage.

Weirdly, on coveralls.io, only Rubinius (rbx-19mode) shows a 1%
lack in coverage due to a line in docile_spec.rb. This should
ensure that the spec files are never measured for code coverage.

spec/spec_helper.rb

@@ -9,7 +9,9 @@ SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
   SimpleCov::Formatter::HTMLFormatter,
   Coveralls::SimpleCov::Formatter
 ]
-SimpleCov.start
+SimpleCov.start do
+  add_filter "/spec/"
+end
 
 test_dir = File.dirname(__FILE__)
 $LOAD_PATH.unshift test_dir unless $LOAD_PATH.include?(test_dir)