Try to enable dashboard (again) (#6069)

* Revert "Revert "Enable the Ray dashboard by default (#5976)" (#6068)"

This reverts commit 1a3e97cf23.

* fix tests that assume the dashboard isn't a job

* travis

python/ray/worker.py

@@ -75,6 +75,11 @@ ERROR_KEY_PREFIX = b"Error:"
 # entry/init points.
 logger = logging.getLogger(__name__)
 
+try:
+    import aiohttp
+except ImportError:
+    aiohttp = None
+
 try:
     import setproctitle
 except ImportError:
@@ -820,8 +825,8 @@ def init(address=None,
          redis_password=None,
          plasma_directory=None,
          huge_pages=False,
-         include_webui=False,
-         webui_host="127.0.0.1",
+         include_webui=aiohttp is not None,
+         webui_host="localhost",
          job_id=None,
          configure_logging=True,
          logging_level=logging.INFO,
@@ -902,8 +907,8 @@ def init(address=None,
         include_webui: Boolean flag indicating whether to start the web
             UI, which displays the status of the Ray cluster.
         webui_host: The host to bind the web UI server to. Can either be
-            127.0.0.1 (localhost) or 0.0.0.0 (available from all interfaces).
-            By default, this is set to 127.0.0.1 to prevent access from
+            localhost (127.0.0.1) or 0.0.0.0 (available from all interfaces).
+            By default, this is set to localhost to prevent access from
             external machines.
         job_id: The ID of this job.
         configure_logging: True if allow the logging cofiguration here.