Ensure the karma and npm commands' exit status is returned by Rake

`system` was being used to run Karma. Although `system` returns false if
the command it executes returns a non-zero exit status, this was not
causing Rake to propagate the failure.

Swapped to use Rake's `sh` call instead.

http://www.ruby-doc.org/stdlib-2.1.2/libdoc/rake/rdoc/FileUtils.html#method-i-sh
http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-system
Merge pull request #9 from moneyadviceservice/propagate_karma_exit_status

Ensure the karma and npm commands' exit status is returned by Rake