Eliminate use of deprecated methods from JRuby

[headius]

In jruby/jruby#9299 I am removing many old deprecated methods from JRuby. StringIO still uses one that is deprecated but not being deleted. Any uses of deprecated APIs in JRuby should be migrate to alternatives that are supported by JRuby 9.4 and later.

• The one API mentioned: jruby/jruby@5d9ebfb

There may be other examples.

[headius]

I've pushed #208 for the one case but there's a number of APIs deprecated in 10.0.x without equivalent APIs in 9.4.x. They'll need to wait.