Fix Heroku related issues

[cmfcmf]

This PR fixes multiple issues related to our Heroku setup:

• addons section of app.json: This section controls which services are spun up for review apps. I removed the sendgrid service, because review apps can just as well use the sendgrid service used by our production app.
• app.json: Instead of sharing SENDGRID_PASSWORD and SENDGRID_USERNAME from our production app, directly share SMTP_* environment variables. These are then picked up by our mailer in app/config/environments/production.rb (up until now, sending emails from review apps didn't work)
• app.json HEROKU_APP_NAME: Requests setting the HEROKU_APP_NAME environment variable to the name of the review app, which can then be used inside app/config/environments/production.rb to configure the mailer. Previously, the mailer of our review apps was not able to generate urls in emails, because it didn't know the host to use.
• app.json DISABLE_DATABASE_ENVIRONMENT_CHECK: Required for Rails 5, allows to execute 'unsafe' migrations on Heroku, even if our application is running in "production".
• README.md: Remove Errbit badge (fixes #692), increase badge maintainability
• travis.yml: Instead of deploying to Heroku from within Travis, use Heroku's functionality to listen for push events.

Please squash-merge this when you merge it :)

[coveralls]

Coverage remained the same at 100.0% when pulling 6a88946477358794f0f864f80abf3166f5e9aad2 on cmfcmf:fix-heroku into 305971d67cdf67eb11ea8e8472968d713ac0e2dc on hpi-schuelerklub:dev.