libtaskotron: terminology changes, yaml format changes

[Lukas Brabec]

New naming convention was introduced in commit [1d21976]. Task YAML files are now called
formulae and key `task:` in task formula was replaced with key `actions:` which better 
describes its meaning.

This change means that task formulae with old 'task:' key won't work and will raise 
TaskotronYamlError (unfortunately at this moment this applies to all tasks in our 
bitbucket repo, but do not worry, patches will come).

LB

[1d21976] https://bitbucket.org/fedoraqa/libtaskotron/commits/1d21976465253e77d4a8d032...