Adding `nvm_version_greater_than_or_equal_to`
Using `nvm_version_greater_than_or_equal_to` instead of `nvm_version_greater` where it increases readability.
Using `nvm_version_greater` and `nvm_version_greater_than_or_equal_to` instead of `expr` in `nvm install`.
Instead of using expr to check if paths need cleaning, compare cleaned paths to original paths and export appropriately.
Merge pull request #528 from creationix/remove_last_expr

Remove last usages of `expr`