Add Support for Hashdiff 1.0.0

When using Hashdiff version 1.0.0, octocatalog-diff failed with the following error:
`uninitialized constant OctocatalogDiff::CatalogDiff::Differ::HashDiff (NameError)`

Hashdiff 1.0.0 was released on July, 15 2019 and replaced the constant "HashDiff" with "Hashdiff".

This commit fixes this error and keeps octocatalog-diff backwards compatible to older versions
of HashDiff.

Author: Stefan Peer

lib/octocatalog-diff/catalog-diff/differ.rb

@@ -11,6 +11,8 @@
 require_relative '../util/util'
 require_relative 'filter'
 
+HashDiff = Hashdiff unless defined? HashDiff
+
 module OctocatalogDiff
   module CatalogDiff
     # Calculate the difference between two Puppet catalogs.