Fixed CacheControl package name

With okhttp 3, the package name changed. In the okhttp3.OkHttpConnector,
the incorrect class is loaded, creating a ClassNotFoundException at
runtime when okhttp3 only is loaded.

Author: alecharp

src/main/java/org/kohsuke/github/extras/okhttp3/OkHttpConnector.java

@@ -1,6 +1,6 @@
 package org.kohsuke.github.extras.okhttp3;
 
-import com.squareup.okhttp.CacheControl;
+import okhttp3.CacheControl;
 import okhttp3.ConnectionSpec;
 import okhttp3.OkHttpClient;
 import org.kohsuke.github.HttpConnector;