Can't install gem json v 1.8.6

I have to work on an old rails 4.2 codebase. I have installed Ruby 2.3 as a snap package - I am trying to avoid rvm.
Now, when I run bundle install it complains that it can't install the json gem. Here is the error message:

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

current directory: /home/user/.gem/gems/json-1.8.6/ext/json/ext/generator
//bin/ruby -r ./siteconf20190430-29073-znwkkd.rb extconf.rb
sh: 1: //bin/ruby: not found

extconf failed, exit code 127

When I try to install just the gem with 'gem install json -v '1.8.6' --source 'https://rubygems.org/'' I get this error:

ERROR: Error installing json:
ERROR: Failed to build gem native extension.

current directory: /home/user/.gem/gems/json-1.8.6/ext/json/ext/generator
/snap/bin/ruby -r ./siteconf20190430-30141-z86zxd.rb extconf.rb
mkmf.rb can't find header files for ruby at /snap/ruby/109/lib/ruby/include/ruby.h

extconf failed, exit code 1

So, as far as I can tell the native extension is looking in the wrong directory for the ruby headers - I guess...

Start by reproducing the reported bundle install and gem install json -v '1.8.6' failures with the Ruby snap package. Compare the /snap/bin/ruby and //bin/ruby paths and the missing /snap/ruby/109/lib/ruby/include/ruby.h headers; done means the json gem installs successfully for this setup.