Added script reference to angular-ui-router.js

Without this change I was getting:
Uncaught Error: [$injector:modulerr] Failed to instantiate module todo due to:
Error: [$injector:modulerr] Failed to instantiate module ionic due to:
Error: [$injector:modulerr] Failed to instantiate module ionic.service due to:
Error: [$injector:modulerr] Failed to instantiate module ionic.service.view due to:
Error: [$injector:modulerr] Failed to instantiate module ui.router due to:
Error: [$injector:nomod] Module 'ui.router' is not available! You either misspelled the module name or forgot to load it. If registering a module ensure that you specify the dependencies as the second argument.