Relax `msgpack` version dependency

[N0xFF]

To fix segmentation fault bug we need to use at least msgpack 1.2.2:

2018-01-11 version 1.2.2:

• Fix bug to occur SEGV occasionally (depends on GC timing) when exttype is used

But transit-ruby locked to 1.1.0:

transit-ruby/transit-ruby.gemspec

Line 30 in 7d77db7

spec.add_dependency "msgpack", "~> 1.1.0"

[semaperepelitsa]

Feel free to use our fork:
https://github.com/nashbridges/transit-ruby (nashbridges#3)