Fix bad path for mocha binary

`require.resolve('mocha')` resoves to 'node_modules/mocha/index.js'
`path.dirname('node_modules/mocha/index.js')` logically resolves to 'node_modules/mocha/'

So, if you add '../.bin' you get 'nodes_modules/.bin' but the mocha binary is served in [the bin directory](https://github.com/visionmedia/mocha/tree/master/bin).