Suppress `warning: BigDecimal.new is deprecated`

[yahonda]

BigDecimal.new is deprecated in Ruby 2.5. https://github.com/ruby/bigdecimal/commit/5337373

• Steps to reproduce:

$ ruby -v
ruby 2.5.0rc1 (2017-12-14 trunk 61243) [x86_64-linux]
$ make check

... snip ...

/path/to/ruby-oci8/test/test_oranumber.rb:705: warning: BigDecimal.new is deprecated
/path/to/ruby-oci8/test/test_oranumber.rb:710: warning: BigDecimal.new is deprecated

... snip ...

• This pull request has been tested with these versions of Ruby:

  ruby 2.5.0dev (2017-12-21 trunk 61391) [x86_64-linux]
  ruby 2.4.3p205 (2017-12-14 revision 61247) [x86_64-linux]
  ruby 2.3.6p384 (2017-12-14 revision 61254) [x86_64-linux]
  ruby 2.2.9p480 (2017-12-15 revision 61259) [x86_64-linux]
  ruby 2.1.9p490 (2016-03-30 revision 54437) [x86_64-linux]
  ruby 2.0.0p648 (2015-12-16 revision 53162) [x86_64-linux]