Upgrade jshint for templates string support

Summary:
Support for template literals was added to jshint in 2.5.0, and 2.5.4 added some
bugfixes for them.

Test Plan:
Ran `npm install` in devtools, then went into webapp

Added this to javascript/shared-package/poppler_test.js

    var x = 7;
    console.log(`${x}`);

and ran ../devtools/khan-linter/runlint.py
javascript/shared-package/poppler_test.js with no errors.

Also ran `make lint` to find any newly introduced errors. Fixes for that in
another diff.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12431