Support for MRI 2.0 as building platform

MRI 2.0 introduced flonums to speed up float operations.

In this mode SYMBOL_FLAG represented differently –
http://rxr.whitequark.org/mri/source/include/ruby/ruby.h?v=2.0.0-p0#416

In this patch I remove hardcoded value for SYMBOL_FLAG on mri and
fallback to original value.
Merge pull request #2180 from brainopia/patch-2

Support for MRI 2.0 as building platform