remove msgpack pin (influxdata#818)

The hard lock prevents this from being co-installed with many other
packages.  For instance, it's preventing it from being included in
openstack (which is on 0.6.2 and working on 1.0.0 now).

Signed-off-by: Matthew Thode <mthode@mthode.org>

Author: prometheanfire

requirements.txt

@@ -2,4 +2,4 @@ python-dateutil>=2.6.0
 pytz
 requests>=2.17.0
 six>=1.10.0
-msgpack==0.6.1
+msgpack