creates postgres extensions if they don't exist

db/migrate/20151028213830_add_unaccent_extension.rb

@@ -1,5 +1,5 @@
 class AddUnaccentExtension < ActiveRecord::Migration
   def change
-    execute "create extension unaccent"
+    execute "create extension unaccent if not exists unaccent"
   end
 end

db/migrate/20151028221647_add_pg_trgm_extension.rb

@@ -1,5 +1,5 @@
 class AddPgTrgmExtension < ActiveRecord::Migration
   def change
-    execute "create extension pg_trgm"
+    execute "create extension pg_trgm if not exists pg_trgm"
   end
 end