use Bluebird in promise utils sleep so it has .done and .nodeify

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/utils.js

@@ -21,6 +21,7 @@ limitations under the License.
  */
 
 const unhomoglyph = require('unhomoglyph');
+import Promise from 'bluebird';
 
 /**
  * Encode a dictionary of query parameters.