Ensure the spring binfile from the correct gem is loaded

It turns out that Gem.bin_path won't return the bin path from the
already-activated spring gem if there is a newer version on the system -
in such a case it'll return the newer one, so we'll end up loading
lib/spring/binstub.rb from the old version, which will then load
bin/spring from the newer version.

So we shouldn't rely on Gem.bin_path here, as we want to load the
bin/spring from the version of the gem which has already been activated.

Fixes #238.

Author: jonleighton

lib/spring/binstub.rb

@@ -1,12 +1,13 @@
-command = File.basename($0)
+command  = File.basename($0)
+bin_path = File.expand_path("../../../bin/spring", __FILE__)
 
 if command == "spring"
-  load Gem.bin_path("spring", "spring")
+  load bin_path
 else
   disable = ENV["DISABLE_SPRING"]
 
   if Process.respond_to?(:fork) && (disable.nil? || disable.empty? || disable == "0")
     ARGV.unshift(command)
-    load Gem.bin_path("spring", "spring")
+    load bin_path
   end
 end