(hotfix) lock in rack dependency for now
(hotfix) lock in rack dependency for now

This is needed to ensure the test suite passes in modern ruby
versions as well as 1.8.7.

We still have a few legacy apps running 1.8.7 that need this and
I want to make sure this gem will run for them for now.  Eventually
we will drop this requirement and test this gem again modern Rack.
But for now I need to continue locking in a 1.8.7 compatible Rack
for testing purposes.

redding/dassets