Refactor `nvm_source` a bit.

install.sh

diff --git a/install.sh b/install.sh
index 9e57a8b..f46f81d 100755
--- a/install.sh
+++ b/install.sh
@@ -22,19 +22,16 @@
   NVM_METHOD="$1"
   if [ -z "$NVM_SOURCE" ]; then
     local NVM_SOURCE
-  else
-    echo "$NVM_SOURCE"
-    return 0
-  fi
-  if [ "_$NVM_METHOD" = "_script" ]; then
-    NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.22.0/nvm.sh"
-  elif [ "_$NVM_METHOD" = "_script-nvm-exec" ]; then
-    NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.22.0/nvm-exec"
-  elif [ "_$NVM_METHOD" = "_git" ] || [ -z "$NVM_METHOD" ]; then
-    NVM_SOURCE="https://github.com/creationix/nvm.git"
-  else
-    echo >&2 "Unexpected value \"$NVM_METHOD\" for \$NVM_METHOD"
-    return 1
+    if [ "_$NVM_METHOD" = "_script" ]; then
+      NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.22.0/nvm.sh"
+    elif [ "_$NVM_METHOD" = "_script-nvm-exec" ]; then
+      NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.22.0/nvm-exec"
+    elif [ "_$NVM_METHOD" = "_git" ] || [ -z "$NVM_METHOD" ]; then
+      NVM_SOURCE="https://github.com/creationix/nvm.git"
+    else
+      echo >&2 "Unexpected value \"$NVM_METHOD\" for \$NVM_METHOD"
+      return 1
+    fi
   fi
   echo "$NVM_SOURCE"
   return 0
@@ -67,7 +64,7 @@
     echo "=> Downloading nvm from git to '$NVM_DIR'"
     printf "\r=> "
     mkdir -p "$NVM_DIR"
-    git clone "$(nvm_source "git")" "$NVM_DIR"
+    git clone "$(nvm_source git)" "$NVM_DIR"
   fi
   cd "$NVM_DIR" && git checkout --quiet v0.22.0 && git branch --quiet -D master >/dev/null 2>&1
   return
@@ -75,9 +72,9 @@
 
 install_nvm_as_script() {
   local NVM_SOURCE
-  NVM_SOURCE=$(nvm_source "script")
+  NVM_SOURCE=$(nvm_source script)
   local NVM_EXEC_SOURCE
-  NVM_EXEC_SOURCE=$(nvm_source "script-nvm-exec")
+  NVM_EXEC_SOURCE=$(nvm_source script-nvm-exec)
 
   # Downloading to $NVM_DIR
   mkdir -p "$NVM_DIR"