Issue #900 - postcss-cssnext

[magsout]

grunt-cssnext is depreacted, so I added postcss-cssnext

Instrcutions:

npm uninstall grunt-cssnext
npm install
make build

r? @miketaylr

Close #900

[miketaylr]

LGTM (everything works as expected locally)

[miketaylr]

(we might have to clear the Travis cache if the tests fail post-merge)

[magsout]

poke @webcompat/collaborators

npm uninstall grunt-cssnext
npm install
make build

cc @webcompat/collaborators @dstreet @StabbyMcDuck @tsah20 @deepthivenkat @cch5ng @dshgna

[miketaylr]

In theory our checkDependencies grunt task should catch this -- something like:

🐓 grunt
Loading "cssnext.js" tasks...ERROR
>> Error: Cannot find module 'postcss-cssnext'

Running "checkDependencies:default" (checkDependencies) task
>> postcss-cssnext: not installed!
>> Invoke npm install to install missing packages
Warning: Task "checkDependencies:default" failed. Use --force to continue.

But not a bad idea to remove grunt-cssnext

[magsout]

Ah yes good point