Fix npm postinstall error ("not a recognized command" running `bower install`)

As mentioned in the wiki article "[Setting up HabitRPG locally](http://habitrpg.wikia.com/wiki/Setting_up_HabitRPG_locally#Install_Node.js_.28includes_npm.29)", `npm install` fails on Windows with the following error:

    '.' is not recognized as an internal or external command, operable program or batch file.

Because the Windows command line sees forward slashes as beginning a command argument, it believes `.` is the name of the executable.  This can be worked around simply by removing the explicit path to Bower.  (npm automatically places commands provided by dependencies in the install environment's `PATH`, so this works even when Bower is not installed globally.)