Install JSDuck 4.6.2 into tools/gem_modules.

https://github.com/senchalabs/jsduck

gem install jsduck\
  --install-dir gem_modules\
  --bindir gem_modules/.bin\
  --no-rdoc\
  --no-ri\
  --env-shebang\
  --include-dependencies

Needed for generating documentation pages for mediawiki-core and
mediawiki-extensions-visualeditor.

Change-Id: Ib72e93f227364f4c32970bd4db850620bb518334