Use mysociety's version of ruby-msg.

A temporary measure until https://github.com/aquasync/ruby-msg/pull/10
can be merged. ruby-msg and sass (>=3.3) both define their own versions
of OrderedHash (sass only uses its version under Ruby 1.8.7) and they
conflict.
Upgrade compass-rails to version 2.0.0

2.0.1 depends on sass 3.3.13 (as described here), which causes segfaults
under ruby 1.8.7 on asset precompilation.
Merge branch 'update-compass-rails-conservatively' into rails-3-develop
Merge branch 'rails-3-develop' of ssh://git.mysociety.org/data/git/public/alaveteli into rails-3-develop