Unlocked Mocha dependency, fixes #68

[lolmaus]

Mocha is now locked at 2.x which is equivalent to >= 2.0.0, < 3.0.0.

[lolmaus]

Build fails due to conflict in Bower's Ember version requirements:

Unable to find a suitable version for ember, please choose one:
    1) ember#^1.10.0 which resolved to 1.13.11 and is required by ember-mocha
    2) ember#>=1.4 <2 which resolved to 1.13.11 and is required by ember-cli-shims#0.0.3
    3) ember#>= 1.8.1 < 2.0.0 which resolved to 1.13.11 and is required by ember-data#1.0.0-beta.19.2
    4) ember#^2.2.0 which resolved to 2.2.0 and is required by ember-test-helpers#0.5.19

[rwjblue]

We no longer consume ember-test-helpers from Bower, so that conflict should be resolved.