Fix grunt-browserify dev dependency; 2.x led to 2.0.x branch being installed, which is not compatible with grunt 4.x, and caused a peer dependency violation. Explicitly installing 2.1.x instead fixes this problem, as 2.1.0 introduced grunt 4.x support.
Merge pull request #22 from joepie91/patch/build-deps

Fix grunt-browserify dev dependency; 2.x led to 2.0.x branch being...