backwards compatible fix for breaking changes introduced in 0.12.1

a change was introduced in Elixir 0.12.1 which broke compilation of
exmsgpack because bitstring modifier args were not being expanded.
This will be fixed in 0.12.2 but this is a backwards compatible patch
that will ensure exmsgpack will work on all previous versions of
Elixir including 0.12.1.

Initial discussion: https://github.com/elixir-lang/elixir/issues/1977
Fix: https://github.com/elixir-lang/elixir/commit/952052869ff7af0e293d2a7160b1aebc68fc46be