Update test and development dependencies to latest versions from npm,
except for PhantomJS. The PhantomJS 1.9.x -> 2.x transition requires
additional changes in tests.

A couple of issues came up during the upgrade:

 - 'constructor' is no longer a valid name for describe() blocks

 - The require() in Karma's Browserify config ended up being resolved
   relative to the location of karma.config.js for reasons I haven't
   debugged, instead of looking in node_modules/. Using
   require.resolve() to get the absolute path works around the issue.