Improving the handling of missing dependencies.

git-svn-id: https://google-api-ruby-client.googlecode.com/svn/trunk@75 c1d61fac-ed7f-fcc1-18f7-ff78120a04ef

examples/sinatra/explorer.rb

@@ -12,6 +12,14 @@ $LOAD_PATH.unshift(lib_dir)
 $LOAD_PATH.uniq!
 
 require 'rubygems'
+begin
+  gem 'rack', '= 1.2.0'
+  require 'rack'
+rescue LoadError
+  STDERR.puts "Missing dependencies."
+  STDERR.puts "sudo gem install rack -v 1.2.0"
+  exit(1)
+end
 begin
   require 'sinatra'
   require 'liquid'