Add version guard around PostgreSQL spec that uses lock_timeout

lock_timeout was added in PostgreSQL 9.3, and Sequel supports back
to PostgreSQL 8.4.

spec/adapters/postgres_spec.rb

@@ -93,7 +93,7 @@ def c.exec_prepared(*); super; nil end
       # Must disconnect to ensure connections with lock/statement timeout are not left in the pool
       @db.disconnect
     end
-  end
+  end if DB.server_version >= 90300
 
   it "should be able to handle various types of IN/NOT IN queries" do
     ds = @db.select(1)