When no `io.js` versions are installed, show `N/A`, not `io.js-`, as the resolved default alias.
When no `io.js` versions are installed, show `N/A`, not `io.js-`, as the resolved default alias.

Fixes #822.

bevacqua/nvm,PeterDaveHello/nvm,Ingelheim/nvm,mirydz/nvm,Theodeus/nvm,domenicosolazzo/nvm,PeterDaveHelloKitchen/nvm,cnpm/nvm,williamboman/nvm,jaawerth/nvm,wahyutaufik/nvm,thomasf/nvm,PeterDaveHelloKitchen/nvm,tianzhihen/nvm,slariviere/nvm,icyflame/nvm,lukechilds/nvm,hanhailong/nvm,creationix/nvm,tianzhihen/nvm,ljharb/nvm,pmq20/nvm,edwmurph/nvm,gtdeng/nvm,woss/nvm,SampleLiao/nvm,devedbox/nvm,Theodeus/nvm,kumaria/nvm,thomasf/nvm,bevacqua/nvm,kumaria/nvm,gtdeng/nvm,PeterDaveHello/nvm,lukechilds/nvm,icyflame/nvm,hanhailong/nvm,cnpm/nvm,youleiy/nvm,edwmurph/nvm,woss/nvm,evocateur/nvm,williamboman/nvm,Richardphp/nvm,Kuzirashi/nvm,ljharb/nvm,JanChw/nvm,Kuzirashi/nvm,jhliberty/nvm,creationix/nvm,Ingelheim/nvm,dfackler/nvm,fnogatz/swivm,SampleLiao/nvm,devedbox/nvm,nontage24/nvm,Richardphp/nvm,nontage24/nvm,tlevine/nvm,tlevine/nvm,mirydz/nvm,elrasguno/nvm,JanChw/nvm,youleiy/nvm,evocateur/nvm,wahyutaufik/nvm,dfackler/nvm,pmq20/nvm,jhliberty/nvm,elrasguno/nvm,jaawerth/nvm,slariviere/nvm,domenicosolazzo/nvm