Update devDependencies to latest versions (#49)

Cannot update chai to 4.2.0 as the new proxies used for property
access are not compatible with the current test environment.

Leaving gulp at stable 3.9.1 release rather than 'next' 4.0.0 which
has breaking changes.

Note phantomjs, used by gulp-mocha-phantomjs and jshint has no
ongoing development so should probably get replaced later:

    ariya/phantomjs#15344

Author: bz2