(PDOC-70) Always generate the JSON doc when running the rake task

I'm super enthusiastic about the potential for folks to build things on
top of the JSON output, we've had several related conversations at
Config Management Camp and on puppet-dev recently. I think this is a
sane default, and calling the file strings.json seems a nice
reinforcement.

lib/puppet-strings/rake_tasks.rb

@@ -6,7 +6,9 @@ require 'puppet_x/puppetlabs/strings/util'
 namespace :strings do
   desc 'Generate Puppet documentation with YARD.'
   task :generate do
-    PuppetX::PuppetLabs::Strings::Util.generate
+    PuppetX::PuppetLabs::Strings::Util.generate([
+      {emit_json: 'strings.json'}
+    ])
   end
 
   desc 'Serve YARD documentation for modules.'