Action Text: bundle package.json in built gem.

After 866da19f we now use package.json
to install Action Text's JS dependencies (see JS_PACKAGE_PATH).

But when the gem pacakge for Action Text is built, package.json was not
included so running `rails action_text:install` in a Rails app would
fail with:

```
rails action_text:install
rails aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - xxx/gems/actiontext-6.0.0.alpha/lib/templates/../../package.json
./bin/rails:4:in `<main>'
Tasks: TOP => app:template
(See full trace by running task with --trace)
```

actiontext/actiontext.gemspec

@@ -17,7 +17,7 @@
   s.email    = ["javan@javan.us", "sstephenson@gmail.com", "david@loudthinking.com"]
   s.homepage = "https://rubyonrails.org"
 
-  s.files        = Dir["CHANGELOG.md", "MIT-LICENSE", "README.md", "lib/**/*", "app/**/*", "config/**/*", "db/**/*"]
+  s.files        = Dir["CHANGELOG.md", "MIT-LICENSE", "README.md", "lib/**/*", "app/**/*", "config/**/*", "db/**/*", "package.json"]
   s.require_path = "lib"
 
   s.metadata = {