[bin] Improve rbenv-install so I can't make silly mistakes

Ref: postmodern/ruby-install#345

bin/rbenv-install

@@ -9,7 +9,7 @@ then
   exit
 fi
 
-if [ "$1" == "ruby" ] && [ ! -z "$2" ]; then
+if [[ "$1" == "ruby" ]] && [[ ! -z "$2" ]]; then
   RUBY_VER="$2"
 
   echo "Note: This is just a small wrapper around 'ruby-install' to prefill the 'install-dir' params for rbenv support"
@@ -19,7 +19,7 @@ if [ "$1" == "ruby" ] && [ ! -z "$2" ]; then
   RBINSTDIR="$HOME/.rbenv/versions/$RUBY_VER"
   RBINSTCMD="ruby-install --install-dir \"$RBINSTDIR\" ruby $RUBY_VER"
 
-  if [ -d "$RBINSTDIR" ]; then
+  if [[ -d "$RBINSTDIR" ]]; then
     echo "It appears that the install directory already exists:"
     echo "  $RBINSTDIR"
     echo
@@ -45,13 +45,19 @@ if [ "$1" == "ruby" ] && [ ! -z "$2" ]; then
     echo "Wasn't ok, so we're quitting. Maybe try using 'ruby-install' directly?"
     exit
   fi
-elif [ "$1" == "versions" ] || [ "$1" == "list" ]; then
-  exec ruby-install
+elif [[ "$1" == "versions" ]] || [[ "$1" == "list" ]]; then
+  # This will download the latest versions, then display them
+  exec ruby-install --latest
+elif [[ "$1" == "update" ]] || [[ "$1" == "--update" ]] || [[ "$1" == "--latest" ]] || [[ "$1" == "-L" ]]; then
+  exec ruby-install --latest
 else
-  echo "This wrapper only handles one small case.. which you didn't hit. Did you want to just use 'ruby-install' directly?"
+  echo "This wrapper only handles a couple of small cases.. which you didn't hit."
+  echo
+  echo "Did you want to just use 'ruby-install' directly?"
   echo
   echo "Usage:"
   echo "  $0 ruby VERSION : Install the given VERSION using 'ruby-install'"
   echo "  $0 versions     : List available versions to install"
+  echo "  $0 update       : Update list of ruby versions"
   exit
 fi