Rise grunt-concurrent limit

[miguelcoba]

From grunt-concurrent docs:

limit

Type: Number
Default: Number of CPU cores (require('os').cpus().length) with a minimum of 2

Limit of how many tasks that are run concurrently.

In machines with few cpus/cores, the default limit of 2 the 'grunt debug' task can't start enough tasks for
starting the node-inspector task, so the command fails and the capp can't be debugged.
Should be increased so that no matter if there are few cpus/cores, the task can run correctly.