Convenience method to monitor custom transactions

Use Appsignal.monitor_transaction to instrument and log errors for
custom actions.
Add option to ignore a specific action
Remove allowed failures from .travis.yml
Add rbx to allowed failures
Show Travis badge for master [skip ci]
Output which Ruby version we're running the specs in
Generate SH script to run all tests

Switching Ruby from Rake turns out not to work, generate a shell script
instead.
Log reason of shutdown
Merge pull request #137 from appsignal/log_shutdown_reason

Log reason of shutdown
Bump to v0.9.2 [ci skip]
Remove ActiveSupport dependency and vendor notifications
Use bash instead of sh in bundle_and_spec_all
Try --login in test runner
Log if appsignal is not active for an environment
Merge pull request #140 from appsignal/log_not_active

Log if appsignal is not active for an environment
Bump to v0.9.3 [ci skip]
Update cacert.pem
Merge pull request #132 from appsignal/cacerts_update

[WIP] Update cacert.pem
Merge pull request #138 from appsignal/no_dependencies

Remove ActiveSupport dependency and vendor notifications
Fix issue where backtrace can be nil