Deprecate running `gulp` commands directly

[UltCombo]

npm run dev provides better modularity (does not require the user to install global packages) and ensures that compatible package versions are used with regard to CLI binaries (currently, gulp and mocha).

The "gulp tasks" section in the readme should be removed and merged into the "npm scripts" section.

The fact that rocksflow uses gulp under the hood should be nothing but an implementation detail.