Suppress warning `BigDecimal.new` is deprecated
Suppress warning `BigDecimal.new` is deprecated

This PR suppresses the following warning.

```console
% ruby -v
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
% RUBYOPT=-w bundle exec rake
(snip)
/home/vagrant/src/ruby-plsql/lib/plsql/oci_connection.rb:298: warning:
BigDecimal.new is deprecated; use Kernel.BigDecimal method instead.
```

rsim/ruby-plsql,yahonda/ruby-plsql,rsim/ruby-plsql,yahonda/ruby-plsql