Tweaking nvm_ls_current tests.
Silencing error output in the "Listing Versions" teardown script.
Removing unnecessary cleanup in "Listing Versions" tests.
Adding `nvm_num_version_groups`
Use `nvm_num_version_groups` in `nvm_ls`
Noting the changed behavior of `nvm ls vx.y.` to return something useful.
Merge pull request #526 from creationix/add_nvm_num_version_groups

Replace `expr` with `nvm_num_version_groups`