New features / Improvements

Added insertTag option to control the placement of the tags option [https://github.com//pull/4008]
Added handler for AJAX errors [https://github.com//issues/3501]
Added insertTag to control the tag position [https://github.com//pull/4008]

Bug fixes

Fixed positioning issues with static dropdown parents [https://github.com//issues/3970]
Fixed existing selections not always being respected with array data [https://github.com//issues/3990]
Sanitize automatically generated ids so CSS identifiers can be used [https://github.com//issues/3618]
Recursively apply defaults so AJAX defaults can be set [https://github.com/select2/select2/commit/983cd8e765c5345bfe7d3bdcc3b0c882a35461ca]
No need to recalculate the top of the dropdown twice [https://github.com//pull/4155]

Updated Bootstrap and Font Awesome dependencies [https://github.com/select2/select2/commit/a5e539b509778eabeb8ce79e191b3ee1e81f6deb, https://github.com/select2/select2/commit/81a4a68b113e0d3e0fb1d0f8b1c33ae1b48ba04f, https://github.com/select2/select2/commit/6369f5f173fb81ec692213782945cc737e248da5]
Use Jekyll's highlighting instead of prettify [https://github.com/select2/select2/commit/54441e6a22be3969dd934ccb769f5d7dde684bfb, https://github.com/select2/select2/commit/74387b98632c75b06d15d83ad5359b9daf0f5dcb, https://github.com/select2/select2/commit/a126b53b4c90fac33b5d855894647cd8bcac3558, https://github.com/select2/select2/commit/75163d67cb80e4279965a97e9eeda5b171806085]
Corrected responsive width example to properly show it working [https://github.com/select2/select2/commit/63d531a9c0ab51f05327492a56f3245777762b45]
Replaced protocol-relative URLs with HTTPS protocol [https://github.com//pull/4127]
Code snippets for mapping id and text [https://github.com//issues/4086]
Document how to trigger change just for Select2 [https://github.com//issues/3620]
Added notes about DOM events [https://github.com/select2/select2/commit/37dbe059fce4578b46b7561e6243b7fdc63ac002]