CI: Coverage fails for node6

[ryan-roemer]

In the istanbul command, only on node6 in Travis, we get:

/home/travis/build/FormidableLabs/builder/node_modules/mkdirp/index.js:92
                if (!stat.isDirectory()) throw err0;
                         ^
TypeError: Cannot read property 'isDirectory' of undefined
    at Function.sync (/home/travis/build/FormidableLabs/builder/node_modules/mkdirp/index.js:92:26)
    at process.<anonymous> (/home/travis/build/FormidableLabs/builder/node_modules/istanbul/lib/command/common/run-with-cover.js:237:28)
    at process.g (events.js:291:16)
    at emitOne (events.js:96:13)
    at process.emit (events.js:188:7)
    at process.exit (internal/process.js:164:15)
    at done (/home/travis/build/FormidableLabs/builder/node_modules/mocha/bin/_mocha:417:32)
    at afterWrite (_stream_writable.js:388:3)
    at _combinedTickCallback (internal/process/next_tick.js:86:20)
    at process._tickCallback (internal/process/next_tick.js:104:9)

Possibly related to: https://github.com/substack/node-mkdirp/pull/110