Bump jQuery version

[SSilence]

github reports two security issues with current jquery version 2.2.4 used in public/package.json:
https://nvd.nist.gov/vuln/detail/CVE-2016-10707
https://nvd.nist.gov/vuln/detail/CVE-2015-9251

[niol]

For CVE-2016-10707, I do not think selfoss does AJAX requests to third party sites, so it should not be vulnerable.
For CVE-2015-9251, selfoss does not seem to use the attr function with third party supplied data.

[jtojnar]

Quoting from #1134 (comment):

There are many breaking changes regarding to Deferred, which will need to be manually checked. And we should probably switch to A+-style promises throughout the codebase.

The remaining deprecations and breaking changes should be addressed in 8e991c4, except for bind() deprecation. That is better left for a separate PR.