Upgrade to Ruby 2.4.0

[jfly]

Ruby 2.4.0 came out in December.

I just upgraded to Ruby 2.4.0 locally, and ran into the following issue while running "bundle install":

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /home/jeremy/.bundle/ruby/2.4.0/gems/json-1.8.3/ext/json/ext/generator
/usr/bin/ruby -r ./siteconf20170120-20980-175rrec.rb extconf.rb
creating Makefile

current directory: /home/jeremy/.bundle/ruby/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR=" clean

current directory: /home/jeremy/.bundle/ruby/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
generator.c: In function ‘generate_json’:
generator.c:861:25: error: ‘rb_cFixnum’ undeclared (first use in this function)
     } else if (klass == rb_cFixnum) {
                         ^~~~~~~~~~
generator.c:861:25: note: each undeclared identifier is reported only once for each function it appears in
generator.c:863:25: error: ‘rb_cBignum’ undeclared (first use in this function)
     } else if (klass == rb_cBignum) {
                         ^~~~~~~~~~
make: *** [Makefile:242: generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /home/jeremy/.bundle/ruby/2.4.0/gems/json-1.8.3 for inspection.
Results logged to /home/jeremy/.bundle/ruby/2.4.0/extensions/x86_64-linux/2.4.0/json-1.8.3/gem_make.out

An error occurred while installing json (1.8.3), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.3'` succeeds before bundling.

rails/rails#27450 looks relevant to this.