work with ember-engines

[arkadiyk]

It fails to build a project if ember-gestures included into Ember Engine because app.bowerDirectory is undefined.

index.js:

    if (!process.env.EMBER_CLI_FASTBOOT) {
      if (app.env === "production") {
        app.import(app.bowerDirectory + '/hammer.js/hammer.min.js');
      } else {
        app.import(app.bowerDirectory + '/hammer.js/hammer.js');
      }
    }