browser-tests - use fixed version of mocha

Not sure if this is a bug or a feature, but in Mocha 1.21 the suite fails in the same manner for all the suites:
(seems https://github.com/visionmedia/mocha/issues/1292 is the one)

`cd browser-tests`

`npm install -g mocha@1.20 ; mocha allTests.js  --reporter spec --browser=phantomjs --framework=agilityjs`

```
  TodoMVC - agilityjs
    No Todos
      √ should hide #main and #footer (406ms)
    New Todo
      √ should allow me to add todo items (656ms)
      √ should clear text input field when an item is added (240ms)
      √ should append new items to the bottom of the list (551ms)
      √ should trim text input (218ms)
      √ should show #main and #footer when items added (406ms)
  ...
```


`npm install -g mocha@1.21 ; mocha allTests.js  --reporter spec --browser=phantomjs --framework=agilityjs`

```
  TodoMVC - agilityjs
    No Todos
      1) "before each" hook
      2) "after each" hook
  ...
```

The error message is "Error: timeout of 0ms exceeded".

Mocha 1.20.1 is the last one with which the suite is working fine.