Add validation to avoid this plugin from being installed on ruby < 1.9.0 && rails < 3.0.0 && redmine < 2.1.0 (Relates #1)

Author: luismaia

README.md

@@ -12,9 +12,15 @@ __Features__:
  * Adds a task which allows the cronjob to fetch emails from all active email configurations.
  * Allows a configuration to be inactivated to stop its emails synchronisation with Redmine.
 
+__Restrictions__:
+
+* The plugin requires a Ruby version equal or higher than 1.9.x.
+* The plugin requires a Rails version equal or higher than 3.x.x.
+* The plugin requires a Redmine version equal or higher than 2.1.0.
+
 __Remarks__:
 
-* The plugin is prepared and intended to run with any IMAP and POP3 email account, however 
+* The plugin is prepared and intended to run with any IMAP and POP3 email account, however
   some issues can accur due to security certificates.
 * When using SSL, please check that the machine has the proper certificates installed
   by running the following terminal commands:

init.rb

@@ -1,13 +1,38 @@
 require 'redmine'
 require 'email_configurations_hooks'
 
-Redmine::Plugin.register :redmine_email_fetcher do
-  name 'Redmine Email Fetcher plugin'
-  author 'Luis Maia'
-  description 'This plugin allow us to configure several IMAP and POP3 email accounts from where emails can be fetch to Redmine'
-  version '0.0.1'
-  url 'https://github.com/luismaia/redmine_email_fetcher'
-  author_url 'https://github.com/luismaia'
-
-  menu :admin_menu, :email_configurations, {controller: 'email_configurations', action: 'index'}, caption: :title_email_configurations
+if RUBY_VERSION < '1.9'
+  ruby_version_error = '*** error redmine_email_fetcher: This plugin (redmine_email_fetcher) was not installed, '\
+  'since it requires a Ruby version equal or higher than 1.9 (and your Ruby version '\
+  "is #{RUBY_VERSION})"
+
+  # Rails.logger.error ruby_version_error
+  puts ruby_version_error
+
+elsif Rails::VERSION::MAJOR < 3
+  rails_version_error = '*** error redmine_email_fetcher: This plugin (redmine_email_fetcher) was not installed, '\
+  'since it requires a Rails version equal or higher than 3 (and your Rails version '\
+  "is #{Rails::VERSION::MAJOR})"
+
+  # Rails.logger.error rails_version_error
+  puts rails_version_error
+
+else
+  Redmine::Plugin.register :redmine_email_fetcher do
+    name 'Redmine Email Fetcher plugin'
+    author 'Luis Maia'
+    description 'This plugin allows the configuration of several IMAP and POP3 email accounts'\
+    'from where emails can be fetch into Redmine'
+    version '0.0.2'
+    url 'https://github.com/luismaia/redmine_email_fetcher'
+    author_url 'https://github.com/luismaia'
+    requires_redmine version_or_higher: '2.1.0'
+
+    menu :admin_menu,
+         :email_configurations,
+         { controller: 'email_configurations', action: 'index' },
+         caption: :title_email_configurations
+  end
+
+  # puts 'This plugin works with the current Ruby, Rails and Redmine versions'
 end