Make the output quiet. relates to AGJS-189

basically, when running the tests, wget and tar like to end up in the stderr stream for some reason,  which then will fail our tests.  we didn't notice this since failOnError was false in grunt-shell.  but will the most recent version it defaults to true.

using a custom callback in grunt-shell and making wget and tar quiet will help this

this needs to be merged before https://github.com/aerogear/aerogear-js/pull/126