Update nvm.sh
Update nvm.sh

Add prompt when there is no binary for node version greater than v1.0.0

Currently, on FreeBSD, if we try `nvm install 4.2`, we will only get:
> Installing node v1.0 and greater from source is not currently supported

In fact I had no idea what's wrong because I didn't use `-s` parameter for it,
and then I found that there are two reasons:
1. There was no pre-built nodejs binary from official for FreeBSD, which means we need to build from source.
2. nvm doesn't support build for v1.0 and greater version from source yet.

So I think there should be a prompt message to tell the user about the reason why nvm will go to build from source,
no matter it will succeed or fail.

lukechilds/nvm,PeterDaveHello/nvm,evocateur/nvm,dfackler/nvm,dfackler/nvm,Kuzirashi/nvm,PeterDaveHelloKitchen/nvm,edwmurph/nvm,PeterDaveHelloKitchen/nvm,Kuzirashi/nvm,slariviere/nvm,evocateur/nvm,tlevine/nvm,creationix/nvm,tlevine/nvm,creationix/nvm,ljharb/nvm,PeterDaveHello/nvm,edwmurph/nvm,slariviere/nvm,lukechilds/nvm,Theodeus/nvm,Theodeus/nvm,ljharb/nvm