Update dependencies in ethpm.json

[dmihal]

Over in trufflesuite/truffle#807 we're discussing adding functionality to Truffle to update ethpm.json when a new package is installed, similar to how NPM works.

It was suggested that some of this functionality make sense in this module. Do you have any thoughts on this?

[cgewecke]

@dmihal I will raise this with @gnidan tomorrow. Sounds good but not sure how delicate things are here and he knows more about this than I do.

[dmihal]

Sounds good, I'm happy to put together a PR if that helps