lock harder on http because of 0.6.0.pre

We noticed a fresh box pulled in 0.6.0.pre of the http gem, causing some bad behavior with reel 0.4.0.

It looks like reel 0.4.0 has a dependency on http of `>= 0.5.0` that would cause this behavior. For now, we should limit this dependency until the pre-release gems are released and we want to move up.