Update march_hare from 2.2.0 to 2.7.0

This version update fixes some bugs (including memory leaks) and improves
connection recovery.
Don't restart errored jobs if they are cancelled

If an error occurs in a cancelled job (for example, we have a bug that causes
the SSH connection to raise an exception when the job is cancelled), we restart
the job instead of cancelling.

Reported in travis-ci/travis-ci#2971.
Add Sentry support
Update travis-support
Don't set Travis::Amqp.config

MarchHare is being used directly in favour of Travis::Amqp.
Require Raven before using it
Only configure Raven if the DSN is set

Raven throws an exception if you set the DSN to something that isn't a URL,
so passing in `nil` won't work.
Use latest Travis::Logger API from travis-support
Add Celluloid mocking workaround

This workaround is taken from celluloid/celluloid#26 and is needed to avoid
infinite loops when using mocking and Celluloid.
Use a StandardError in the specs instead of Exception

We no longer capture Exception, but StandardError instead.

Also added a text so that it's easier to find this exception if the specs failed
and the exception get raised to the top level (previously it would just complain
about an exception "Exception" being raised with no other information).
Remove trailing whitespace
Add basic specs for dispatcher command
Require Celluloid in actor_mocking support file

Celluloid might not be required depending on load order.
Require travis/support/amqp instead of march_hare
Merge branch 'hh-fix-cancel-restart' into staging