Always download the common test suite before running tests

In #377 I made it possible to run the tests without having internet
access. But it seems that in the process I also disabled the code that
downloads the common test suite when you first check out the
codebase. That's bad, but what makes it worse is that travis hasn't
been running the common test suite because of it.

This changes the behaviour so that the tests download the common test
suite (but does not update it) before running. If you don't have
internet access it continues without it. To update the common test
suite to the latest version, you now need to run
`UPDATE_TEST_SUITE=true rake download_common_tests`.

Rakefile

@@ -4,16 +4,18 @@ require 'rake/testtask'
 
 Bundler::GemHelper.install_tasks
 
-desc "Updates the json-schema common test suite to the latest version"
-task :update_common_tests do
+desc "Downloads the json-schema common test suite"
+task :download_common_tests do
+  update_test_suite = !!ENV['UPDATE_TEST_SUITE']
+
   unless File.read(".git/config").include?('submodule "test/test-suite"')
     sh "git submodule init"
   end
 
   puts "Updating json-schema common test suite..."
 
   begin
-    sh "git submodule update --remote --quiet"
+    sh "git submodule update --quiet" + (update_test_suite ? " --remote" : "")
   rescue StandardError
     STDERR.puts "Failed to update common test suite."
   end
@@ -63,6 +65,4 @@ Rake::TestTask.new do |t|
   t.test_files = FileList.new('test/*_test.rb')
 end
 
-task update: [:update_common_tests, :update_meta_schemas]
-
-task :default => :test
+task :default => [:download_common_tests, :test]