Provide prepend alias for FilterChain@unshift

Ruby 2.5 introduces this alias for all arrays.

lib/rackstash/filter_chain.rb

@@ -251,6 +251,7 @@ module Rackstash
       end
       self
     end
+    alias prepend unshift
 
     # Returns an Array representation of the filter chain.
     #

spec/rackstash/filter_chain_spec.rb

@@ -455,6 +455,11 @@ describe Rackstash::FilterChain do
 
       expect { filter_chain.unshift }.to raise_error ArgumentError
     end
+
+    it 'can use #prepend alias' do
+      filter_chain.prepend filter
+      expect(filter_chain[0]).to eql filter
+    end
   end
 
   describe '#to_a' do