Fix getLevels code

Fix for

````
TypeError: Arguments to path.resolve must be strings
    at Object.exports.resolve (path.js:313:15)
    at Array.map (native)
    at MAKE.decl.getLevels (/Users/tadatuta/tmp/project-stub/.bem/make.js:65:18)
    at res.(anonymous function) [as getLevels] (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/inherit/lib/inherit.js:28:32)
    at registry.decl.setBemBuildNode (/Users/tadatuta/tmp/project-stub/node_modules/bem/lib/nodes/bundle.js:195:30)
    at registry.decl.create-deps.js-node (/Users/tadatuta/tmp/project-stub/node_modules/bem/lib/nodes/bundle.js:370:21)
    at registry.decl.createTechNode (/Users/tadatuta/tmp/project-stub/node_modules/bem/lib/nodes/bundle.js:81:24)
    at null.<anonymous> (/Users/tadatuta/tmp/project-stub/node_modules/bem/lib/nodes/bundle.js:59:37)
    at Array.map (native)
    at null.<anonymous> (/Users/tadatuta/tmp/project-stub/node_modules/bem/lib/nodes/bundle.js:58:29)
    at makePromise.apply (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/q/q.js:927:26)
    at makePromise.promise.promiseDispatch (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/q/q.js:634:41)
    at Object._onImmediate (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/q/q.js:1245:25)
    at processImmediate [as _immediateCallback] (timers.js:330:15)
From previous event:
    at Cmd.exports.Cmd.Cmd._do (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/coa/lib/cmd.js:424:14)
    at Cmd.exports.Cmd.Cmd.do (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/coa/lib/cmd.js:471:17)
    at Cmd.exports.Cmd.Cmd.run (/Users/tadatuta/tmp/project-stub/node_modules/bem/node_modules/coa/lib/cmd.js:459:22)
    at Object.<anonymous> (/Users/tadatuta/tmp/project-stub/node_modules/bem/bin/bem:6:23)
    at node.js:901:3
````