Simplify Purchase.grouped_payable_by_person

Also write a couple of tests for it.
Removed a lot of duplication in Notifier

Also, added tests for notifier
Tidy up Notifier

* Omit `{|format| format.text }` block
* Only call `subject.deliver` for the relevant tests
Simplify Notifier method generation

I removed the lambda for generating the debt notification methods, since
@koronen pointed out that it was superfluous. I could just use `last_updated_by`
in all of the debt notifications, since it will be set to the same person
anyway.
only iterate on the actions for purchase notifier methods
Merge pull request #121 from datasektionen/improve-gpa

Improve code quality a bit
Make Travis CI only build `master`

The settings for cashflow on travis-ci.org is set to only build pull requests.

These two in combination should make Travis build all pull requests, and any
pushes to master, including when pull requests get merged into master.
add 2.1.0 to travis-ci config
Make TravisCI report coverage to CodeClimate
test on 2.1.5 instead of 2.1.0
allow code_climate to connect through webmock
Sort Travis CI config keys lexicographically
Route Travis CI build to container-based infrastructure

See
<http://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure>.
Merge pull request #123 from datasektionen/vk-travis-sudo-false

Route Travis CI build to container-based infrastructure
Sort Travis CI config keys lexicographically

Take II.
bump redcloth version
Remove debts feature

The debts feature has never been used, so I feel that it's really time to remove
it now. It's had its chance for a time in the spotlight now, but apparently it
wasn't very important, and now it is time for it to go away.
Delete redundant initializer

The default locale is set in `config/application.rb`.
Merge pull request #124 from datasektionen/remove-debts

remove debts feature
stub specs for moving purchase-observer behaviour to controller