Removing the slack webhook url check, since we can use this other slack compatible APIs

Ref - ashwanthkumar/gocd-slack-build-notifier#36 and ashwanthkumar/gocd-slack-build-notifier#23

Author: ashwanthkumar

src/main/java/in/ashwanthkumar/slack/webhook/Slack.java

@@ -22,8 +22,6 @@ public class Slack {
     public Slack(String webhookUrl) {
         if (isEmpty(webhookUrl)) {
             throw new IllegalArgumentException("Webhook url is not provided");
-        } else if (!webhookUrl.startsWith("https://hooks.slack.com/services/")) {
-            throw new IllegalArgumentException("Slack Webhook url starts with https://hooks.slack.com/services/");
         }
         this.webhookUrl = webhookUrl;
     }

src/test/java/in/ashwanthkumar/slack/webhook/SlackTest.java

@@ -40,9 +40,4 @@ public void shouldThrowIllegaArgumentExceptionWhenWebHookUrlIsNotGiven() {
         new Slack(null);
     }
 
-    @Test(expected = IllegalArgumentException.class)
-    public void shouldThrowIllegalArgumentExceptionWhenUrlIsNotInSlacWebHookFormat() {
-        new Slack("http://www.foo.com/webhooks");
-    }
-
 }