Add swap file to machines

Used the guide found here:

https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-14-04

The `chmod 0600` is because anyone but root being able to read a
swapfile would mean they could access any pages of memory being stored
there. BAD!
Upgrade dokku version to 0.3.7

In order to upgrade we need to checkout the new version and run `make
install` in its directory. I've moved the install out to a handler so
that this will run any time that the version of the dokku git repo
changes.
Install dokku-slack plugin

This plugin required having environment variables exported in
/home/dokku/dokkurc. The commit gives each plugin an env key which can
be used to define key/val pairs that will be exported. Global env
variables can be set through an env key on the role, this can be used to
set DOKKU_TRACE.

This also introduces the first vault file for storing the slack_url
variable, which points at the slack incoming webhook url.