title: SublimeLinter-Rubocop not running even when enabled and rubocop in path
body: <p>I'm trying to get Rubocop warnings to show as code marks in Sublime Text 3. I'm using <code>SublimeLinter</code> and the <code>SublimeLinter-rubocop</code> package. (The similar <code>RuboCop</code> package does work but doesn't seem to support code markings.)</p>
  
  <p><code>rubocop</code> is installed and runs from the rbenv shims folder:</p>
  
  <pre><code>$ rubocop response_test.rb
  ...
  1 file inspected, 11 offenses detected
  </code></pre>
  
  <p>SublimeLinter is using the rbenv shims:</p>
  
  <pre><code>SublimeLinter: computed PATH using /bin/bash:
  /Users/andrew/.rbenv/shims
  /usr/local/bin
  /usr/bin
  /bin
  /usr/sbin
  /sbin
  </code></pre>
  
  <p>The extension is enabled:</p>
  
  <pre><code>...
  "linters": {
      "rubocop": {
          "@disable": false,
          "args": [],
          "excludes": []
      },
      ...
  }
  ...
  </code></pre>
  
  <p>But the warnings don't show and the linter doesn't even appear to run:</p>
  
  <pre><code>reloading settings Packages/User/SublimeLinter.sublime-settings
  SublimeLinter: ruby: response_test.rb ['/usr/bin/ruby', '-wc'] 
  SublimeLinter: ruby output:
  Syntax OK
  </code></pre>
  
  <p>Is there anything that could account for this? I'm new to Rubocop and Sublime Text so I may have missed something basic. </p>