gem install fpm
Building native extensions.  This could take a while...
ERROR:  Error installing fpm:
	ERROR: Failed to build gem native extension.

    current directory: /usr/local/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
/usr/local/opt/ruby/bin/ruby -r ./siteconf20161226-62014-1huvp4l.rb extconf.rb
creating Makefile

current directory: /usr/local/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR=" clean

current directory: /usr/local/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
generator.c:861:25: error: use of undeclared identifier 'rb_cFixnum'
    } else if (klass == rb_cFixnum) {
                        ^
generator.c:863:25: error: use of undeclared identifier 'rb_cBignum'
    } else if (klass == rb_cBignum) {
                        ^
...
6 warnings and 2 errors generated.
make: *** [Makefile:242: generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /usr/local/lib/ruby/gems/2.4.0/gems/json-1.8.3 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0/json-1.8.3/gem_make.out