GlobalSession::Keystore.create_keypair(:EC) does not work under ruby2.4.x

[mark-dotson-rs]

To repro:

docker run -it rightscale/ops_ruby24x_build:latest /bin/bash
gem install global_session
irb
irb(main):001:0> require 'rubygems'
=> false
irb(main):002:0> require 'global_session'
=> true
irb(main):003:0> keypair = GlobalSession::Keystore.create_keypair(:EC)
TypeError: no implicit conversion of Integer into String
	from /usr/lib/ruby/gems/2.4.0/gems/global_session-3.3.2/lib/global_session/keystore.rb:76:in `generate'
	from /usr/lib/ruby/gems/2.4.0/gems/global_session-3.3.2/lib/global_session/keystore.rb:76:in `create_keypair'
	from (irb):3
	from /usr/bin/irb:11:in `<main>'