Cloud not install with Redmine 2.4.2

Install plugin failed with Redmine 2.4.2 cause of gem's conflict.

Following modification make backlogs installl for RM 2.4.2.
But it may break for other version.

```
--- a/Gemfile
+++ b/Gemfile
@@ -10,7 +10,7 @@ RAILS_VERSION_IS_3 = rails3 =~ deps['rails']

 gem "holidays", "~>1.0.3"
 gem "icalendar"
-gem "nokogiri", "< 1.6.0"
+gem "nokogiri"
 gem "open-uri-cached"
 gem "prawn"
 gem 'json'
```
