Browserify v5

* Upgrades us to browserify v5. This is a breaking change b/c browserify now outputs a buffer instead of a string in its callback!
* tests are now faster b/c we don't use timeouts to wait for the file system, we just use our callback
* tests now check for strings in the expect output instead of comparing a whole file. This approach was brittle and prone to breaking when a transform or browserify changed