Retain Grunt 0.3 support

With the addition of support for Grunt 0.4, projects which are using Grunt 0.3 can no longer be deployed. This does not need to be the case as grunt-cli is capable of running Grunt 0.3 and Grunt 0.4 projects.

I do not see why Grunt 0.3 projects should not be supported by this buildpack (especially since supporting 0.3 is so easy).