Update `nvm` to v0.15.0.

Fixes `nvm copy-packages`; adds `$NVM_SYMLINK_CURRENT` env var to
prevent creation of a "current" symlink; makes `sha1sum` have a higher
priority than `shasum`.
Merge pull request #372 from ljharb/update_to_nvm_0_15_0

Update `nvm` to v0.15.0