Don't allow installing already installed version.
Don't allow installing already installed version.

Trying to install already installed version causes misleading message
from curl and nvm:

    $ nvm install v0.4.11
    curl: (33) HTTP server doesn't seem to support byte ranges. Cannot
    resume.
    nvm: install v0.4.11 failed!

Of course, resuming interrupted installs is still possible (script
checks for existance of `$VERSION` in `$NVM_DIR`, not `$NVM_DIR/src`).

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