Bump msgpack to 1.2.10

This saves some space by matching the version used by Gitaly. List of
changes:

* https://github.com/msgpack/msgpack-ruby/blob/master/ChangeLog
* https://github.com/msgpack/msgpack-ruby/compare/v1.2.6..v1.2.10

Gemfile.lock

@@ -505,7 +505,7 @@ GEM
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
     minitest (5.11.3)
-    msgpack (1.2.6)
+    msgpack (1.2.10)
     multi_json (1.13.1)
     multi_xml (0.6.0)
     multipart-post (2.0.0)