Try to load the (maybe) active libv8 gem on build

Sometimes your rubygems install can get into a 
state where the gem activation succeeds, but the
require will ultimately fail. Make sure that the
gem is actually available before trying to build
with it.

ext/v8/build.rb

@@ -4,6 +4,7 @@ LIBV8_COMPATIBILITY = '~> 3.11.8'
 def have_rubygem_libv8?
   require 'rubygems'
   gem 'libv8', LIBV8_COMPATIBILITY
+  require 'libv8'
   return true
 rescue Gem::LoadError
   return false