Run all guard tests on lib directory changes, too

Guardfile

@@ -4,7 +4,7 @@ guard :minitest do
   watch(%r{^app/controllers/application_controller\.rb$}) { 'test/controllers' }
   watch(%r{^app/controllers/(.+)_controller\.rb$})        { |m| "test/integration/#{m[1]}_test.rb" }
   watch(%r{^app/views/(.+)_mailer/.+})                   { |m| "test/mailers/#{m[1]}_mailer_test.rb" }
-  watch(%r{^lib/(.+)\.rb$})                               { |m| "test/lib/#{m[1]}_test.rb" }
+  watch(%r{^lib/(.+)\.rb$})                               { 'test' }
   watch(%r{^test/.+_test\.rb$}) { 'test' }
   watch(%r{^test/test_helper\.rb$}) { 'test' }
 end