Remove grunt-concurrent

grunt-concurrent poses issues for low memory VM's and cloud instances.

Trying to perform a build on a 512MB cloud server fails; spooling up imagemin, svgmin, and autoprefixer all concurrently pushes beyond the resource ceiling, killing the concurrent task and aborting the build all-together. This is in spite of Ghostium not having any assets for imagemin or svgmin to process by default. And, that said, it shouldn't be much of a build time/performance hit to run these tasks consecutively instead of concurrently...