Use rb_type for type detection in mochilo_pack_one

In ruby 2.x Floats are immediate objects and do not have a separate
type, this caused segfaults as Mochilo was not checking for FLONUM_P
before calling `BUILTIN_TYPE` on the immediate float which tries to
dereference the object.

This changes the logic to simply use `rb_type` which eliminates the need
for special cases for immediate objects, it handles immediate objects
already and falls back to `BUILTIN_TYPE`.
Ugh. Hard tabs.
Whitespace. ;_;
Remove duplicated Rbx specific code
Fucking hard tabs
Merge remote-tracking branch 'origin/master' into flonum-ruby-2
Merge pull request #8 from adelcambre/flonum-ruby-2

Use rb_type for type detection in mochilo_pack_one