Fix extra new line when customising gemfiles

When using `customize_gemfiles`, two new line characters end up being
inserted after the block comment, but it should only be one. This trims
out the output to remove it.

Fixes thoughtbot#216.

lib/appraisal/customize.rb

@@ -1,7 +1,7 @@
 module Appraisal
   class Customize
     def initialize(heading: nil, single_quotes: false)
-      @@heading = heading
+      @@heading = heading&.chomp
       @@single_quotes = single_quotes
     end