Add io.js support to `nvm_version_dir`

nvm.sh

@@ -144,6 +144,8 @@ nvm_version_dir() {
   NVM_WHICH_DIR="$1"
   if [ -z "$NVM_WHICH_DIR" ] || [ "_$NVM_WHICH_DIR" = "_new" ]; then
     echo "$NVM_DIR/versions/node"
+  elif [ "_$NVM_WHICH_DIR" = "_iojs" ]; then
+    echo "$NVM_DIR/versions/io.js"
   elif [ "_$NVM_WHICH_DIR" = "_old" ]; then
     echo "$NVM_DIR"
   else

test/fast/Unit tests/nvm_version_dir

@@ -5,6 +5,7 @@ die () { echo $@ ; exit 1; }
 . ../../../nvm.sh
 
 [ "$(nvm_version_dir)" = "$NVM_DIR/versions/node" ] || die '"nvm_version_dir" did not return new dir path'
+[ "$(nvm_version_dir iojs)" = "$NVM_DIR/versions/io.js" ] || die '"nvm_version_dir iojs" did not return iojs dir path'
 [ "$(nvm_version_dir new)" = "$(nvm_version_dir)" ] || die '"nvm_version_dir new" did not return new dir path'
 [ "$(nvm_version_dir old)" = "$NVM_DIR" ] || die '"nvm_version_dir old" did not return old dir path'
 [ "$(nvm_version_dir foo 2>&1)" = "unknown version dir" ] || die '"nvm_version_dir foo" did not error out'