Update package.json to remove tv4

[davecarlson]

tv4 doesn't seem to be used at all in the source code, so removing the dependancy.

[jonnor]

tv4 is used indirectly. It is an optional dependency in fbp, but browserify resolves all requires it finds, and will thus fail if the module is not present. Mentioned in the commit that introduced it, 2be4f462991166d82e52894beac805c45d40078e

We could maybe move it to a devdep though? Since browserify is just used for generating the distributed .js files