No need to install grunt globally

grunt is listed in our devDependencies already.

Our CONTRIBUTING.md was instructing people to also install grunt
globally. This isn't necessary. Each project can use its own
version of grunt. Only grunt-cli needs to be installed globally.

Author: ericwilligers

CONTRIBUTING.md

@@ -7,7 +7,7 @@
 1. `git submodule update --init --recursive` (Necessary for running tests.)
 1. Install [node](https://nodejs.org/en/) and make sure `npm` is in your $PATH
 1. Run `npm install` in the respository to pull in development dependencies.
-1. Run `npm install -g grunt grunt-cli` to get the build tools for the command line.
+1. Run `npm install -g grunt-cli` to get the build tools for the command line.
 
 ### Contributing