Add --no-summary flag to haml-lint

Haml-lint now prints a summary at the end of the run. E.g.

  11 files inspected, 1 lint detected

We need to disable that so that overcommit can parse the output
correctly.

Here's some more context: sds/haml-lint#176

This will help resolve some of the issues seen in #483.

config/default.yml

@@ -288,6 +288,7 @@ PreCommit:
     description: 'Analyze with haml-lint'
     required_executable: 'haml-lint'
     install_command: 'gem install haml-lint'
+    flags: ['--no-summary']
     include: '**/*.haml'
 
   HardTabs: