Bump immutables dependency

Fuyukai · 1st1 · commit 646308c55912 · 2018-07-30T16:38:49.000+01:00
0.6 has a crash fix.
diff --git a/setup.py b/setup.py
@@ -17,7 +17,7 @@
     packages=['contextvars'],
     provides=['contextvars'],
     install_requires=[
-        'immutables==0.5',
+        'immutables==0.6',
     ],
     license='Apache License, Version 2.0',
     classifiers=[
