JENKINS-25558: Notifications Plugin does not honor the "http_proxy" environment variable: Fix

This commit partially fixes the problem by using an HTTP Proxy without authentication. In
addition, it does not provide test cases, but it does work properly. The message after using
a new build of this plugin was as follows:

Started by user mdesales
Notifying endpoint 'HTTP:https://zapier.com/hooks/catch/o54xyk/'
Building in workspace /app/installs/jenkins/jobs/Status Notification/workspace
Notifying endpoint 'HTTP:https://zapier.com/hooks/catch/o54xyk/'
Finished: SUCCESS

(Sorry, in a rush to get things done and I can try providing a test with an internal
 http proxy server).

*	modified:   src/main/java/com/tikal/hudson/plugins/notification/Protocol.java
- Verifying if the environment variable "http_proxy" is set. If so, decorate the connection
  with the proxy information before making the HTTP request.