fix #177, add a lock file for n

before the version installed, touch a file named `n.lock` in version directory, and util the version has been installed/downloaded locally, remove the file.

Then for that checking function, just check the n.lock of the corresponding version directory is existed.
/cc @tjwebb
busted: I blamed instead of `blame`d. could very well be.

@yorkie feel free to use whichever style you prefer as long as its approximately consistent.