Update code for MaxMind DB version 4 compatibility

Author: laurentpellegrino

src/main/java/co/ipregistry/datasets/IpregistryDataset.java

@@ -8,7 +8,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
-import java.util.Date;
+import java.time.Instant;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutionException;
@@ -86,10 +86,10 @@ public void close() throws IOException {
      *
      * @return when the dataset file in use was created.
      */
-    public Date getBuildDate() {
+    public Instant getBuildDate() {
         Reader readerInstance = reader.get();
         if (readerInstance != null) {
-            return readerInstance.getMetadata().getBuildDate();
+            return readerInstance.getMetadata().buildTime();
         }
         return null;
     }

src/test/java/co/ipregistry/datasets/IpregistryGeolocationDatasetTest.java

@@ -18,7 +18,7 @@ public void test() throws IOException {
                              System.getenv("IPREGISTRY_DATASETS_SECRET_KEY")).build()) {
 
             Assertions.assertTrue(
-                    dataset.getBuildDate().toInstant().isAfter(
+                    dataset.getBuildDate().isAfter(
                             Instant.now().minus(2, ChronoUnit.DAYS)));
 
             GeolocationData data = dataset.lookup("8.8.8.8");