Upgrade to rspec 3.0 gem

use defined `mock` variable instead of calling `double`
get rid of a `should_receive`
rewrite `publish_event` and `publish_events` matchers
fix with new webmock
use `a_hash_including` instead of `hash_including`
fix sqlite rollback error
reword some tests
remove transitional `config.treat_symbols_as_metadata_keys_with_true_values = true` from config
Fix issues related to --order random

make publishable_spec set `@@called` to false before every test
make subscribable_spec clear all TEST_MODE_SUBSCRIBERS before every test
make specs run in random order by default
bump version to 0.8.3 due to matcher changes