Fix if syntax in the bin/mocha-casperjs script

In Ubuntu, /bin/sh has changed from bash to dash, and the mocha-casperjs script produces a few of these errors:
`mocha-casperjs: 3: [: ==: unexpected operator`
due to [this syntax difference](https://wiki.ubuntu.com/DashAsBinSh#A.5B)

Using a single = works in both bash and dash correctly.