Separate runtime dependencies and development dependencies in all the plugins gemspec

All the dependencies for the plugins are declared as runtime dependencies.
But in reality some of the dependencies are only used to make the test run.

One of the gem with the problem is the ruby filter (https://github.com/logstash-plugins/logstash-filter-ruby)

``` ruby
# Gem dependencies
s.add_runtime_dependency 'logstash', '>= 1.4.0', '< 2.0.0'
s.add_runtime_dependency 'logstash-filter-date'
```