Remove '--gruntfile' and argument from inherited options

Command line args are passed forward to the child grunt processes - however if the parent gruntfile is explicitly set by the --gruntfile option on the command line, the child grunt process seems to use the first gruntfile specified - in this case the parent gruntfile.

See [this gist](https://gist.github.com/benwhite-ext-nokia/273286a2f03a94ab89a1) for an example - (subdir_Gruntfile.js needs moving to subdir/Gruntfile.js relative to the other parts)

Running the following from the root results in 'Lookout, a badger!' rather than 'Hello world' as expected:

    grunt --gruntfile /full/path/to/root/Gruntfile.js test-subdir

The attached commit resolves this.