server.js: check existence of files, not directories

The directories exists, what does not exist is the specific
files for the libraries. Now it loads correctly the `npm` modules.
plugin/requiremx: dns in the right scope

The call was returning a `''` because `dns` was not in the scope, so
it did not fired the `resolveMx`.
Merge pull request #22 from ghostbar/hotfix/requiremx

plugin/requiremx: dns in the right scope
Merge pull request #21 from ghostbar/hotfix/loadnpm

server.js: check existence of files, not directories