Fix for ruby 2.4

[praiskup]

FYI:
Upstream dropped rb_cFixnum constant, and moved to rb_cInteger, so to fix builds we had to apply downstream patch:

http://pkgs.fedoraproject.org/cgit/rpms/postgresql-plruby.git/commit/?id=4c94c436be1a1c94183fc43706c633bfe4fb0bf5

[praiskup]

To ease google searching, this is the gcc error:

conversions/basic/conversions.h:5:51: error: 'rb_cFixnum' undeclared (first use in this function)
     rb_hash_aset(plruby_classes, INT2NUM(OIDOID), rb_cFixnum);
                                                   ^~~~~~~~~~
conversions/basic/conversions.h:5:51: note: each undeclared identifier is reported only once for each function it appears in