Use proper @Nullable annotation from spotbugs

pom.xml

@@ -59,6 +59,11 @@
             <scope>compile</scope>
             <optional>true</optional>
         </dependency>
+      <dependency>
+        <groupId>com.github.spotbugs</groupId>
+        <artifactId>spotbugs-annotations</artifactId>
+        <version>4.2.0</version>
+      </dependency>
     </dependencies>
 
     <build>

src/main/java/com/github/steveice10/packetlib/Client.java

@@ -1,7 +1,7 @@
 package com.github.steveice10.packetlib;
 
 import com.github.steveice10.packetlib.packet.PacketProtocol;
-import com.sun.istack.internal.Nullable;
+import javax.annotation.Nullable;
 import java.net.InetSocketAddress;
 
 /**