remove usage of deprecated Futures.catching from Guava

The overload of this method that does not take an Executor as an
argument is removed from Guava starting in release 26.0.

For anyone using helios-testing in a project that uses Guava >= 26.0,
this code in helios-testing causes NoSuchMethodErrors at runtime.

This is related to #1257 which I think missing this `Futures` method.

helios-system-tests/src/main/java/com/spotify/helios/system/SystemTestBase.java

@@ -48,7 +48,6 @@
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.google.common.base.CharMatcher;
 import com.google.common.base.Charsets;
-import com.google.common.base.Function;
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -59,6 +58,7 @@
 import com.google.common.io.Files;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.MoreExecutors;
 import com.google.common.util.concurrent.Service;
 import com.spotify.docker.client.DefaultDockerClient;
 import com.spotify.docker.client.DockerCertificates;
@@ -1019,12 +1019,8 @@ public DeploymentGroupStatus call() throws Exception {
 
   protected <T> T getOrNull(final ListenableFuture<T> future)
       throws ExecutionException, InterruptedException {
-    return Futures.catching(future, Exception.class, new Function<Exception, T>() {
-      @Override
-      public T apply(final Exception ex) {
-        return null;
-      }
-    }).get();
+    return Futures.catching(future, Exception.class, ex -> null, MoreExecutors.directExecutor())
+        .get();
   }
 
   protected static void removeContainer(final DockerClient dockerClient, final String containerId)

helios-testing/src/main/java/com/spotify/helios/testing/HeliosSoloDeployment.java

@@ -25,7 +25,6 @@
 import static java.util.Collections.singletonList;
 
 import com.google.common.annotations.VisibleForTesting;
-import com.google.common.base.Function;
 import com.google.common.base.Optional;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
@@ -34,6 +33,7 @@
 import com.google.common.net.HostAndPort;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.MoreExecutors;
 import com.spotify.docker.client.DefaultDockerClient;
 import com.spotify.docker.client.DockerClient;
 import com.spotify.docker.client.DockerHost;
@@ -592,12 +592,8 @@ public Boolean call() throws Exception {
 
   private <T> T getOrNull(final ListenableFuture<T> future)
       throws ExecutionException, InterruptedException {
-    return Futures.catching(future, Exception.class, new Function<Exception, T>() {
-      @Override
-      public T apply(final Exception ex) {
-        return null;
-      }
-    }).get();
+    return Futures.catching(future, Exception.class, ex -> null, MoreExecutors.directExecutor())
+        .get();
   }
 
   /**