Version bump 1.3.1

Author: eddiezane

src/main/java/com/sendgrid/SendGrid.java

@@ -26,7 +26,7 @@
 import org.apache.http.entity.ContentType;
 
 public class SendGrid {
-  private static final String VERSION           = "1.3.0";
+  private static final String VERSION           = "1.3.1";
   private static final String USER_AGENT        = "sendgrid/" + VERSION + ";java";
 
   private static final String PARAM_TO          = "to[%d]";