Gravity Forms Survey Addon 4.0.0

- Added GPL to plugin header
- Updated Plugin URI and Author URI to use https

- Added "value" modifier to Likert field merge tag to return column text only.

- Added Chinese (China) translation. Credit: Edi Michael.
- Updated minimum Gravity Forms version to 2.0.
- Updated strings for translations.
- Updated Likert CSS for better mobile display.
- Updated low resolution PNG icons with scaleable vector graphics (SVG).
- Fixed Survey field settings referencing incorrect icon URLs.
- Fixed an issue with the Likert field markup.
- Updated inline image in rank field so that it is applied via CSS instead.
- Fixed an issue with the locations the gform_merge_tags hook was being used to include the merge tags.