curl: fix handling of some parameters

 - $verifypeer/$verifyhost/$measureresponsetime: properly handle undef vs. false
 - $matches: default value (array with empty hash) creates bad config (match
   block with missing values)
 - $matches: match blocks are optional if measureresponsetime is enabled
Fix Puppet deprecation warning

Example:

```
Warning: Variable access via 'port' is deprecated. Use '@port' instead. template[/etc/puppet/modules/collectd/templates/plugin/statsd.conf.erb]:6
   (at /etc/puppet/modules/collectd/templates/plugin/statsd.conf.erb:6:in `result')
Warning: Variable access via 'timerpercentile' is deprecated. Use '@timerpercentile' instead. template[/etc/puppet/modules/collectd/templates/plugin/statsd.conf.erb]:21
   (at /etc/puppet/modules/collectd/templates/plugin/statsd.conf.erb:21:in `result')
```
Merge pull request #205 from petems/patch-1

Fix Puppet deprecation warning
Add cpufreq plugin doc
Add cpufreq plugin
Fix travis by using plabs gemfile
Merge pull request #208 from pdxcat/fix_travis2

Fix travis by using plabs gemfile
Merge pull request #206 from muling-tt/master

Add cpufreqplugin
Merge pull request #201 from bogus-py/curl

curl: fix handling of some parameters
Uses the LoadPlugin syntax with bracket when supported

This is fully backwards compatible.

The version check is done exactly how it's supposed to ; I checked the
sources of collectd to be sure there's no conflict.

Globals has always defaulted to false so I'm making it default to false
in this module. I'm not setting it undef because that's a boolean and we
got issues with checking if a boolean is false or undef (see #182).
Cleaning the now redundant LoadPlugin declaration
Add rspec test to ensure the bracket syntax is using when available