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
lots of obnoxious debug statements
weuntho
nteuhonteuhnt