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
```

jruby/jruby#4293

Author: yahonda

lib/plsql/jdbc_connection.rb

@@ -223,8 +223,6 @@ def prepare_call(sql, *bindvars)
     end
 
     RUBY_CLASS_TO_SQL_TYPE = {
-      Fixnum => java.sql.Types::INTEGER,
-      Bignum => java.sql.Types::INTEGER,
       Integer => java.sql.Types::INTEGER,
       Float => java.sql.Types::FLOAT,
       BigDecimal => java.sql.Types::NUMERIC,