Fix `nvm_ls` when `$NVM_DIR` has spaces in it.
Fix `nvm_ls` when `$NVM_DIR` has spaces in it.

Mostly fixes #861.

creationix/nvm,PeterDaveHello/nvm,evocateur/nvm,lukechilds/nvm,bevacqua/nvm,PeterDaveHelloKitchen/nvm,evocateur/nvm,dfackler/nvm,lukechilds/nvm,jaawerth/nvm,tlevine/nvm,PeterDaveHello/nvm,ljharb/nvm,Theodeus/nvm,edwmurph/nvm,slariviere/nvm,slariviere/nvm,Kuzirashi/nvm,creationix/nvm,tlevine/nvm,dfackler/nvm,jaawerth/nvm,ljharb/nvm,Kuzirashi/nvm,bevacqua/nvm,PeterDaveHelloKitchen/nvm,Theodeus/nvm,thomasf/nvm,fnogatz/swivm,thomasf/nvm,edwmurph/nvm