Use `receive_snapshot` to send notifications to Slack.

This will send one message with an extract of the information rather than sending one message per change/improvement.
Remove unused tests.
Fix `new_constants_selector` method name.
Add test for the snapshot formatters.
Remove repo dependency.
Make sure the format for the Slack notifications is correct.
Set the right encoding for the Slack tests on 1.9.3.
Merge pull request #40 from codeclimate/slack_snapshot_notification

Use `receive_snapshot` to send notifications to Slack.