Try a longer timeout

The timeout is causing a few sporadic failures on the CI. Since this
didn't happen previously (when we waited infinitely), I think/hope
having a longer timeout value will solve it.

Author: jonleighton

lib/spring/client/run.rb

@@ -7,7 +7,7 @@ module Client
     class Run < Command
       FORWARDED_SIGNALS = %w(INT QUIT USR1 USR2 INFO WINCH) & Signal.list.keys
       CONNECT_TIMEOUT   = 1
-      BOOT_TIMEOUT      = 10
+      BOOT_TIMEOUT      = 20
 
       attr_reader :server