Use Gemfile for development deps rather than gemspec.

- To make it work properly for all platforms I had to
  include conditionals in the gemspec. That's pretty
  weird as the gemspec needs to be eventually compiled
  into a static file at gem release time.
- Relax version specifiers to use `>=` rather than `~>`.
  While the latter is the recommended way, I want to be
  able to use `bundle update` and update to the absolute
  latest version of a gem w/o having to research if a new
  minor or major version has come out. Plus, the Gemfile.lock
  is in source to ensure gems aren't accidentally updated to
  a new version when not intended.

This means that `gem install vcr --development` won't work
but I've never heard of anyone doing that. All VCR
contributors use bundler, anyway.
Update to latest version of all gems.
Add missing require.
Add changelog entry for current exxon version support.
Recent versions of MultiJson allow yajl to throw ArgumentError for encoding issues.
Remove shoulda cuke.

- It doesn't give us any test value, really; it was
  just there for the documentation.
- I've never heard a single report of any VCR users
  using shoulda, so I'm not convinced we need the cuke.
- It's fairly obvious how to integrate VCR w/ shoulda.
- Upgrading should pulled in active support 4.0, which
  won't install on < 1.9.3, so to keep building against
  older versions of ruby it's best to drop the shoulda
  duke and dependency.
Don't unnecessarily load bundler in rakefile.

This allows --standalone to be used instead.
Update typhoes_old gem file.
Lock timecop to a version that can install on 1.8.7.
For now disable json_pure test due to odd bug.
Fix 1.8.7 build.

Apparently multi_json w/ yajl raises a different error
on 1.8.7 here :(.
Merge pull request #329 from vcr/update_all_gems

Update all gems