Async is less reliable in zsh versions < 5.0.8

`stty` occasionally hangs (always in CircleCI) inside the async pty.

Disable the tests for now until we can figure out and fix/workaround
this issue.

spec/terminal_session.rb

@@ -18,6 +18,10 @@ 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