Fixes some weird travis failures noted on master and in #236

lib/rspec/expectations/differ.rb

@@ -16,6 +16,9 @@ def diff_as_string(input_data_new, input_data_old)
         file_length_difference = 0
         diffs.each do |piece|
           begin
+            if data_old == []
+              data_old = [""]
+            end
             hunk = Diff::LCS::Hunk.new(
               data_old, data_new, piece, context_lines, file_length_difference
             )