Rise gunt-concurrent task limit.

[miguelcoba]

Fixes issue with default value for grunt-concurrent limit in machines with few cpu/cores.

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.

Fixes issue #144