diff --git a/setup.py b/setup.py
index 2ef58a0a..8af80030 100644
--- a/setup.py
+++ b/setup.py
@@ -75,11 +75,11 @@
     ],
 
     install_requires=[
-        "greenlet==0.4.10",
-        "requests==2.2.1",
-        "pycontracts==1.7.6",
-        "gevent==1.1.2",
-        "redis==2.9.1",
-        "six==1.9.0"
+        "greenlet",
+        "requests",
+        "pycontracts",
+        "gevent",
+        "redis",
+        "six",
     ],
 )
diff --git a/snowplow_tracker/_version.py b/snowplow_tracker/_version.py
index 02e250a1..8fe87197 100644
--- a/snowplow_tracker/_version.py
+++ b/snowplow_tracker/_version.py
@@ -20,6 +20,6 @@
 """
 
 
-__version_info__ = (0, 8, 0)
+__version_info__ = (0, 9, 0)
 __version__ = ".".join(str(x) for x in __version_info__)
 __build_version__ = __version__ + ''
diff --git a/snowplow_tracker/tracker.py b/snowplow_tracker/tracker.py
index a6830a68..8cab5028 100644
--- a/snowplow_tracker/tracker.py
+++ b/snowplow_tracker/tracker.py
@@ -626,16 +626,16 @@ def track_unstruct_event(self, event_json, context=None, tstamp=None):
     track_self_describing_event = track_unstruct_event
 
     @contract
-    def flush(self, async=False):
+    def flush(self, flush_async=False):
         """
             Flush the emitter
 
-            :param  async:  Whether the flush is done asynchronously. Default is False
-            :type   async:  bool
+            :param  flush_async:  Whether the flush is done asynchronously. Default is False
+            :type   flush_async:  bool
             :rtype:         tracker
         """
         for emitter in self.emitters:
-            if async:
+            if flush_async:
                 emitter.flush()
             else:
                 emitter.sync_flush()