`rbenv install` with no arguments defaults to `rbenv local`

bin/rbenv-install

@@ -66,7 +66,13 @@ for option in "${OPTIONS[@]}"; do
 done
 
 unset VERSION_NAME
+
+# The first argument contains the definition to install. If the
+# argument is missing, try to install whatever local app-specific
+# version is specified by rbenv. Show usage instructions if a local
+# version is not specified.
 DEFINITION="${ARGUMENTS[0]}"
+[ -n "$DEFINITION" ] || DEFINITION="$(rbenv local 2>/dev/null || true)"
 [ -n "$DEFINITION" ] || usage 1