Remove DockerDateFormat

This library was originally using Java 7. Java 7 had `Date` objects
with only millisecond resolution.

This was a problem when translating timestamps with nanosecond
precision from the Docker API to `Date`.

This library is on Java 8 now which supports nanosecond resolution.

This PR fixes the findbugs error here #1068.
See build logs https://travis-ci.org/spotify/docker-client/jobs/426781589.

findbugs-exclude.xml

@@ -5,10 +5,4 @@
     <Bug pattern="SE_BAD_FIELD"/>
   </Match>
 
-  <Match>
-    <Class name="com.spotify.docker.client.DockerDateFormat" />
-    <Method name="clone" />
-    <Bug pattern="CN_IDIOM_NO_SUPER_CALL" />
-  </Match>
-
 </FindBugsFilter>

src/main/java/com/spotify/docker/client/ObjectMapperProvider.java

@@ -75,7 +75,6 @@ public Object apply(final Object input) {
       OBJECT_MAPPER.registerModule(MODULE);
       OBJECT_MAPPER.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
       OBJECT_MAPPER.setSerializationInclusion(JsonInclude.Include.NON_NULL);
-      OBJECT_MAPPER.setDateFormat(new DockerDateFormat());
     } catch (Throwable t) {
       log.error("Failure during static initialization", t);
       throw t;