Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- https://github.com/zdharma/fast-syntax-highlighting/commit/ca2e18bbc9e27b9264206c257d2ab68838162c70
- http://www.zsh.org/mla/users/2018/msg00424.html

spec/terminal_session.rb

@@ -18,10 +18,6 @@ class TerminalSession
     tmux_command("new-session -d -x #{opts[:width]} -y #{opts[:height]} '#{cmd}'")
   end
 
-  def zsh_version
-    @zsh_version ||= Gem::Version.new(`#{ZSH_BIN} -c 'echo -n $ZSH_VERSION'`)
-  end
-
   def tmux_socket_name
     @tmux_socket_name ||= SecureRandom.hex(6)
   end