Properly fix libedit support

Bug #70 came back because in 8b1a8d6 we
started loading the application via Process.spawn rather than via
spawning, and therefore the require of the readline library went back to
coming after the pgid is set.

This time I managed to actually figure out the root cause of the issue.

libedit's implementation of rl_initialize() calls tcsetattr(), which
causes the kernal to generate a SIGTTOU because we cannot write to the
terminal after having set the pgid. The default action of SIGTTOU is to
stop the process, which is why it hangs.

Therefore the proper solution is to just ignore the SIGTTOU.

OMG I am so happy I worked this out.

Author: jonleighton

lib/spring/application/boot.rb

@@ -5,6 +5,7 @@
   Spring::JSON.load(ENV.delete("SPRING_ORIGINAL_ENV").dup)
 )
 Signal.trap("TERM") { app.terminate }
+Signal.trap("TTOU", "IGNORE")
 
 app.preload if ENV.delete("SPRING_PRELOAD") == "1"
 app.run

lib/spring/boot.rb

@@ -1,10 +1,6 @@
 require "socket"
 require "thread"
 
-# readline must be required before we setpgid, otherwise the require may hang,
-# if readline has been built against libedit. See issue #70.
-require "readline"
-
 require "spring/configuration"
 require "spring/env"
 require "spring/process_title_updater"