Get NPM scripts to work cross-platform

[brcrista]

These scripts were broken on macOS / Linux: In Windows CMD shells, single & means "run A then B", like a semicolon. In Bash shells, single & means "run A in the background."

In both shells, double && means "run A then run B if A was successful." I believe this is what we want. Tested in PowerShell as well.

A possibly better fix would be to move the npm install step to the Gulpfile so it doesn't have to be repeated every time, but this is the quicker fix in line with the current design.