Trim trailing whitespace

If a license text had blank lines, they would result in a trailing
whitespace when prefixed with the line comments.

This change removes trailing whitespace from the middle lines,
leaving top and bottom as is.

Fixes #10

testdata/expected/file.el

@@ -1,11 +1,11 @@
 ;; Copyright 2016 Google Inc.
-;; 
+;;
 ;; Licensed under the Apache License, Version 2.0 (the "License");
 ;; you may not use this file except in compliance with the License.
 ;; You may obtain a copy of the License at
-;; 
+;;
 ;;      http://www.apache.org/licenses/LICENSE-2.0
-;; 
+;;
 ;; Unless required by applicable law or agreed to in writing, software
 ;; distributed under the License is distributed on an "AS IS" BASIS,
 ;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.