update grunt-concurrent package

- without this update, an `npm shrinkwrap` of a package with `gruntfile-gtx` as a dependency (and a newer version of `grunt` installed?) will fail with the following:

        npm ERR! Error: Problems were encountered
        npm ERR! Please correct and try again.
        npm ERR! missing: grunt@~0.4.0, required by grunt-concurrent@0.4.3
        npm ERR!     at shrinkwrap_ (/usr/local/lib/node_modules/npm/lib/shrinkwrap.js:30:15)
        npm ERR!     at /usr/local/lib/node_modules/npm/lib/shrinkwrap.js:24:5
        npm ERR!     at /usr/local/lib/node_modules/npm/lib/ls.js:44:30
        npm ERR!     at /usr/local/lib/node_modules/npm/node_modules/read-installed/read-installed.js:130:5
        npm ERR!     at /usr/local/lib/node_modules/npm/node_modules/read-installed/read-installed.js:258:14
        npm ERR!     at cb (/usr/local/lib/node_modules/npm/node_modules/slide/lib/async-map.js:48:11)
        npm ERR!     at /usr/local/lib/node_modules/npm/node_modules/read-installed/read-installed.js:258:14
        npm ERR!     at cb (/usr/local/lib/node_modules/npm/node_modules/slide/lib/async-map.js:48:11)
        npm ERR!     at /usr/local/lib/node_modules/npm/node_modules/read-installed/read-installed.js:258:14
        npm ERR!     at cb (/usr/local/lib/node_modules/npm/node_modules/slide/lib/async-map.js:48:11)
        npm ERR! If you need help, you may report this *entire* log,
        npm ERR! including the npm and node versions, at:
        npm ERR!     <http://github.com/npm/npm/issues>

        npm ERR! System Darwin 13.1.0
        npm ERR! command "node" "/usr/local/bin/npm" "shrinkwrap"
        npm ERR! cwd /Volumes/samara/projects/decipher/core/beacon
        npm ERR! node -v v0.10.28
        npm ERR! npm -v 1.4.9
        npm ERR!
        npm ERR! Additional logging details can be found in:
        npm ERR!     /Volumes/samara/projects/decipher/core/beacon/npm-debug.log
        npm ERR! not ok code 0

    It's a mystery to me why this would happen exactly, but the problem may be either the mismatched versions of `grunt-concurrent` in `gruntfile-gtx`'s deps/devDeps, or with `grunt-concurrent` itself.

    Anyway, this fixes the issue.
Merge pull request #16 from boneskull/master

update grunt-concurrent package