Finds NVM_DIR without using cd

Using `cd` makes the nvm directory the number-one directory in `autojump`'s database---which is somewhat confusing since the user basically never `cd`s to it directly!---so here's an alternative method that doesn't use the `cd` command. I've checked it works in both Bash and Zsh.
Use only shell builtins to avoid triggering autojump, instead of needing GNU readlink

Under Zsh, the `-q` flag to `cd` makes it not run `chpwd` hooks and hence makes it not add `~/.nvm` to `autojump`.
Merge pull request #294 from 00Davo/patch-1

Finds NVM_DIR with `cd -q` on zsh