Memoize request to registry

[Jesús Leganés-Combarro]

Since npm-remote-ls fetch the info for all the versions of a module stored on the registry there's no need to ask for it each time, so memoize/cache the results and use them instead of doing a new request. The async module already being used has a function just for it.

[Benjamin E. Coe]

@piranna want to draft up a pull? would happily accept it.