Only use the typed data interface for Ruby version >= 2.2.0

[methodmissing]

Addresses concern in onelogin#10 (comment)

Gist of the change:

• Ruby exposes a RUBY_API_VERSION_CODE constant in ruby/version.h
• I asserted the versions that includes the changeset that introduces typed_data to the openssl extension ( ruby/ruby@163cb5b )

Lourenss-Air:ruby lourens$ git tag --contains 163cb5b43de68cef7ae4fa053d710098399d1359
v2_2_0
v2_2_0_preview2
v2_2_0_rc1
v2_2_1
v2_2_2
v2_2_3
v2_2_4
v2_2_5
v2_3_0
v2_3_0_preview1
v2_3_0_preview2
v2_3_1

I think it's OK to skip v2_2_0_preview2 and v2_2_0_rc1 and standardise on v2_2_0 as the version that should start using the typed_data interface for this extension.

@pallan @csfrancis @jnormore @stonith

[jtdowney]

I tested on a few versions of ruby from 1.9.3 through 2.3.1 and it worked correctly.