Update matcher failure messages
Support Berkshelf 3.0

Berkshelf 3 replaces `Berksfile#install(path: path)` with `Berksfile#vendor(path)`

Signed-off-by: Seth Vargo <sethvargo@gmail.com>
Use RSpec's `before(:suite)` and `after(:suite)` for Berkshelf
Reduce calls to ChefSpec::Berkshelf.setup! to improve speed
Allow overriding automatic attributes
Add #at_compile_time and #at_converge_time resource predicates
Update CHANGELOG to reflect the most recent additions to the beta
Version bump to v3.0.0.beta.2