build: run-ci makefile rule

Adding a single rule to be called from Jenkins.

Jenkins jobs typically call:
python ./configure
make -j $(getconf _NPROCESSORS_ONLN)
make test-ci

After this change, we can have Jenkins call:
make run-ci -j $(getconf _NPROCESSORS_ONLN)

This allows us to customize how we call configure
for different repos or branches (e.g. joyent\node).

PR-URL: https://github.com/nodejs/io.js/pull/2134


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>