Merge pull request influxdata#472 from influxdata/cache-version

Cache version per influxdb instance and reduce ping() calls for every…

src/main/java/org/influxdb/impl/InfluxDBImpl.java

@@ -71,6 +71,7 @@ public class InfluxDBImpl implements InfluxDB {
   private final InetAddress hostAddress;
   private final String username;
   private final String password;
+  private String version;
   private final Retrofit retrofit;
   private final InfluxDBService influxDBService;
   private BatchProcessor batchProcessor;
@@ -313,7 +314,10 @@ public Pong ping() {
 
   @Override
   public String version() {
-    return ping().getVersion();
+    if (version == null) {
+      this.version = ping().getVersion();
+    }
+    return this.version;
   }
 
   @Override