Fix errors launching protractor from Grunt on Windows.

This resolves the following issue with running grunt tests on Windows:

grunt test
...
Running "shell:protractor_update" (shell) task
>> 'node_modules\protractor\bin\webdriver-manager' is not recognized as an internal or external command,
>> operable program or batch file.
Warning: Done, with errors. Use --force to continue.

Aborted due to warnings.