Rise gunt-concurrent task limit.

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