Temporarily add submodules to the Rails repo for Arel and Rack::Mount. These submodules will be removed once we add proper Gemfile following to bundler

Revert "Split arel_table into method to get a relation and another to memoize the default relation."

This reverts commit bd517908.

Ruby 1.9.2: disallow explicit coercion via method_missing. Only give friendly nil errors for Array and Active Record methods.

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#3451 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>