Improve error message when target returns no data.

If an incorrect target was specified, an empty dataset would be
retreived, which in turn made the check fail:

```
ALERT - [aaa-01.lpa-b/graphite-es-heap-used] - Check failed to run: undefined method `empty?'
for nil:NilClass, ["/etc/sensu/plugins/graphite-data.rb:226:in `eval'", "(eval):1:in
`value_to_check'", "/etc/sensu/plugins/graphite-data.rb:226:in `eval'",
"/etc/sensu/plugins/graphite-data.rb:226:in `value_to_check'",
"/etc/sensu/plugins/graphite-data.rb:122:in `run'",
"/opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-plugin-0.3.0/lib/sensu-plugin/cli.rb:56:in `block in '"] .
```

This fixes it by shortcircuiting when no (or nil) data is recieved, giving a helpful critical
instead.
update changelog
Merge branch 'master' into fix_check_graphite

Conflicts:
	CHANGELOG.md
Merge pull request #43 from ministryofjustice/fix_check_graphite

Improve graphite-data.rb error message when target returns no data.
Add Trusty support
Merge pull request #44 from ministryofjustice/map_jinja_fixes

URGENT: Add Trusty support
Re-pin Uchiwa version back to 0.2.4-1

The 0.3 (golang) release of uchiwa appears to need Websockets support
in proxies in front of it. Without this, it gives a '404' error..

While this is fixed, re-pin the version back down to the Node 0.2.4 version.
Merge pull request #45 from ministryofjustice/repin_uchiwa

URGENT: Re-pin Uchiwa version back to 0.2.4-1
Bump version
Fixes issues with uchiwa 0.3.2

Previous PR shipped uchiwa 0.3.2 however some behaviour skipped
my attention. Uchiwa uses websockets so we need to get our nginx
to upgrade the connection to Websocket when proxying. Also uchiwa
needs to point to public HTML directory via -p switch.
Make sure sensu.conf nginx config is no longer present after upgrade

Also updated formula-requirements dependencies
Upgrade uchiwa to 0.3.2