Support Ruby 2.7

[k0kubun]

This pull request fixes the following compilation error on Ruby master (newer than ruby/ruby@50f5a0a):

../../../../ext/attribute_builder/attribute_builder.cc: In function ‘attributes_type normalize_data(VALUE)’:
../../../../ext/attribute_builder/attribute_builder.cc:16:28: error: invalid conversion from ‘int (*)(...)’ to ‘int (*)(VALUE, VALUE, VALUE) {aka int (*)(long unsigned int, long unsigned int, long unsigned int)}’ [-fpermissive]
 #define FOREACH_FUNC(func) reinterpret_cast<int (*)(ANYARGS)>(func)
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../ext/attribute_builder/attribute_builder.cc:130:25: note: in expansion of macro ‘FOREACH_FUNC’
   rb_hash_foreach(data, FOREACH_FUNC(normalize_data_i),
                         ^~~~~~~~~~~~
In file included from /home/k0kubun/.rbenv/versions/ruby/include/ruby-2.7.0/ruby/ruby.h:2127:0,
                 from /home/k0kubun/.rbenv/versions/ruby/include/ruby-2.7.0/ruby.h:33,
                 from ../../../../ext/attribute_builder/attribute_builder.cc:1:
/home/k0kubun/.rbenv/versions/ruby/include/ruby-2.7.0/ruby/intern.h:534:6: note:   initializing argument 2 of ‘void rb_hash_foreach(VALUE, int (*)(VALUE, VALUE, VALUE), VALUE)’
 void rb_hash_foreach(VALUE, int (*)(VALUE, VALUE, VALUE), VALUE);
      ^~~~~~~~~~~~~~~

ref: ruby/ruby#2404

Caveats: This patch does not work for 2.7.0-preview1...

[coveralls]

Coverage increased (+0.1%) to 99.162% when pulling 7e1c84e on k0kubun:ruby27 into 4f2a60a on eagletmt:master.

[sodabrew]

Since ruby/ruby@a569bc0 what should C++ extensions do as a best practice now?

[k0kubun]

Here would not be a good place to ask it. Please go to ruby/ruby#2404 or https://bugs.ruby-lang.org.