Issue title: Slower than mocha-phantomjs
 Issue body: I've recently migrated to testem ci from mocha-phantomjs. And it looks like testem ci is 3 times slower.
Do you have any idea why this would be the case?

 Comments: 
 Comment 0: I have found that the console messages slow down the phantom testing to the point of running out of memory in high test counts. I killed console in my client host file and it's way faster. Do you have many console messages?
 Comment 1: >  I killed console in my client host file

Do you mean js console or command line shell? How did you kill it?
 Comment 2: stuck this in my client.mustache

``` js
// Kill console message in testing mode
(function() {
    var method;
    var noop = function () {};
    var methods = [
      'assert',
      'clear',
      'count',
      'debug',
      'dir',
      'dirxml',
      //'error',
      'exception',
      'group',
      'groupCollapsed',
      'groupEnd',
      'info',
      'log',
      'markTimeline',
      'profile',
      'profileEnd',
      'table',
      'time',
      'timeEnd',
      'timeStamp',
      'trace',
      'warn'
    ];
    var length = methods.length;
    var console = (window.console = window.console || {});

    while (length--) {
      method = methods[length];
      console[method] = noop;
    }
}());
```

Kills everything other than error. Not ideal, but helps.

{'source': 'GithubIssues', 'repo_name_with_owner': 'testem/testem', 'issue_id': 134461724, 'repo_stars': 2950.0}