Update dependency ranges

[whymarrh]

Context: https://github.com/ethereumjs/keythereum/blob/4fc9842a42c40bdc78821b9c4badd1714223f1a4/package.json#L28-L35

All the dependencies listed in the package.json file are fixed to exact versions. Is there a particular reason why this is? For a library package, keythereum should allow minor and patch versions to be used (and allow npm to deduplicate them in consuming applications).