title: Middleman 4.0.0 with HAML template engine gives error when starting with server command
body: <p>When executing 'middleman server' I get the following errors:</p>
  
  <p>Haml::TempleEngine: Option :context is invalid
  Haml::TempleEngine: Option :outvar is invalid</p>
  
  <p>I tried updating middleman amongs other things.
  This is my current gemfile:</p>
  
  <pre><code>source "https://rubygems.org"
  gem "middleman", "&gt;= 4.0.0"
  gem "middleman-compass", "&gt;= 4.0.0"
  gem "jquery-middleman"
  gem "middleman-sprockets", "&gt;= 4.0.0.rc.1"
  gem "rubocop"
  gem "rubocop-junit-formatter"
  gem "middleman-s3_sync"
  gem "middleman-dotenv"
  </code></pre>
  
  <p>Haven't worked with middleman before so I would love some constructive advice. </p>