Better checking for existence of Rake tasks

With the current setup, a broken Rakefile will cause all tasks to be
considered undefined. This silently fails under most situations, for
example, pushing a Rails 3 app with a broken Rakefile will cause
asset precompilation to not work, with no error during the push.

Conflicts:
	lib/language_pack/ruby.rb
don't error out if there's no Gemfile
add bin to PATH for rake task check
ruby 1.8.7 does not have rake available
update changelog
v70