Using debugger-ruby_core_source fails to build native gem extension

I just ran into an issue where my Rails app would no longer bundle, and I couldn't get it running. It all looks good up until it gets to debugger-ruby_core_source and it fails to build gem native extension. If you've come across a similar problem you might be looking at something like this.

Fetching gem metadata from https://rubygems.org/.........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
...
Using debugger-linecache 1.2.0
Using debugger-ruby_core_source 1.3.5

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

    /Users/Dwight/.rvm/rubies/ruby-2.1.2/bin/ruby extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/Users/Dwight/.rvm/rubies/ruby-2.1.2/bin/ruby

/Users/Dwight/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/fileutils.rb:1573:in `stat': No such file or directory @ rb_file_s_stat - ./212/ruby_debug.h (Errno::ENOENT) from /Users/Dwight/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/fileutils.rb:1573:in `block in fu_each_src_dest'
from /Users/Dwight/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/fileutils.rb:1587:in `fu_each_src_dest0' from /Users/Dwight/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/fileutils.rb:1571:in `fu_each_src_dest'
from /Users/Dwight/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/fileutils.rb:399:in `cp' from extconf.rb:83:in `block in <main>'
from extconf.rb:82:in `each' from extconf.rb:82:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /Users/Dwight/.rvm/gems/ruby-2.1.2/gems/    debugger-1.6.8 for inspection.
Results logged to /Users/Dwight/.rvm/gems/ruby-2.1.2/extensions/x86_64-darwin-13/2.1.0    -static/debugger-1.6.8/gem_make.out
    An error occurred while installing debugger (1.6.8), and Bundler cannot continue.
    Make sure that `gem install debugger -v '1.6.8'` succeeds before bundling.

Turns out, this is simply a result of the debugger gem not supporting Ruby 2+. I find it odd that it is therefore still included with the Gemfile that ships with Rails, but nonetheless if you want to replace the gem you might look at debugger2 or Pry.