Use index ascending/descending constants

Author: estolfo

examples/index.rb

@@ -1,5 +1,5 @@
 # Create a single field index
-result = client[:restaurants].indexes.create(cuisine: 1)
+result = client[:restaurants].indexes.create(cuisine: Mongo::Index::ASCENDING)
 
 # Create a compound index
-result = client[:restaurants].indexes.create(cuisine: 1, zipcode: -1)
+result = client[:restaurants].indexes.create(cuisine: 1, zipcode: Mongo::Index::DESCENDING)