Make auto-npm install's when package.json changes

Using empty target trick (see comment)
Make creates build/ if not there, rm from git

Same empty target trick as parent commit
README now says just need to install Node and make
Makefile just uses -p to safely create build/

mkdir -p is in POSIX:
https://github.com/mathquill/mathquill/commit/5378645d955be2bbdb1136db409b2ca0844fb486#makefile-P29
Merge pull request #212 from mathquill/build.empty-targets

Use empty target trick in Makefile