fixed the leveldown error when running block.test.js by installing level-browsify

For installing level-browsify, due to
https://github.com/Level/level-browserify/issues/7 these were some steps:
- at root of the project, "npm install levelup --save"
- then clone level-browserify and then updated package.json to use
leveldown 1.0.0.  Then cd level-browserify && npm install.
Then at root of this project, "npm link ./level-browserify (or
path to wherever level-browserify is)"