Fix multi-line formatting

FIxes the issue described in adamzapasnik#130. It seems that `embedTextToDoc` is returning a string and not an object with a `parts` array.

lib/formatter.js

@@ -19,7 +19,7 @@ const formatMultilineExpressions = (tokens, options, embedTextToDoc) => {
         try {
           if (!formatRaw) {
             formattedExpression = embedTextToDoc(expression, { ...options, parser: 'ruby' });
-            formattedExpression.parts.pop(); // removes newline at the end
+            formattedExpression = formattedExpression.trim(); // removes newline at the end
           }
         } catch (error) {
           formatRaw = true;