Fixnum and Bignum are deprecated in Ruby 2.4
Fixnum and Bignum are deprecated in Ruby 2.4

This pull request addresses these warnings.
https://travis-ci.com/github/rsim/ruby-plsql/jobs/483335621

```
/home/travis/build/rsim/ruby-plsql/lib/plsql/jdbc_connection.rb:225: warning: constant ::Fixnum is deprecated
/home/travis/build/rsim/ruby-plsql/lib/plsql/jdbc_connection.rb:226: warning: constant ::Bignum is deprecated
```

https://github.com/jruby/jruby/issues/4293

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