Update ruby-tty-screen to 0.8.0.
## [v0.8.0] - 2020-05-28
### Added
* Add #windows? platform check
* Add #command_exist? to see if an executable exists before running it
* Add performance tests
### Changed
* Change #jruby? method to hoist within module
* Change #size_from_win_api to check only on windows platform and
hoist definition within module
* Change #size_from_java to hoist within module
* Change #size_from_ioctl to:
* check solaris-like system
* scan all streams to see if any attached to a terminal
* hoist definition within module
* Change #size_from_io_console to perform check on JRuby as well
* Change #size_from_readline to attempt to load readline gem
* Change #run_command to execute command directly without sub shell or temp files
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