When a `go-npm`-enabled module is installed, `go-npm` will pick a destination directory for said module's binary based on the output of `npm bin`. However, if there's no `node_modules` sub-directory present (which is the case when `npm` hoists the dependencies of such module up), the installation directory suggested by `npm bin` doesn't exist and the installation fails.
This fix ensures that the installation directory exists.
`bin` property of package.json must point to a binary that will
be copied by NPM into the NPM Installation's bin directory. This check
was assuming users would point to a directory containing the binary and
not the binary itself
