increased EXPIRY_MARGIN_IN_MINUTES in GitHubClient from 3 to 5 mins (#121)

Author: martidis

src/main/java/com/spotify/github/v3/clients/GitHubClient.java

@@ -67,7 +67,7 @@
  */
 public class GitHubClient {
 
-  private static final int EXPIRY_MARGIN_IN_MINUTES = 3;
+  private static final int EXPIRY_MARGIN_IN_MINUTES = 5;
 
   private Tracer tracer = NoopTracer.INSTANCE;