add rake jquery3 to verify that specs pass with jquery3 also

[tongueroo]

So you probably want straight close this PR w/o merging it. I just thought it was really good that the specs pass with jquery3 also! Figured I'll open this in case someone else was wondering the the same thing.

There's probably a cleaner way to test against different versions of jquery3 and I'm happy to hear suggestions! Otherwise it might make sense to just close this w/o merging it.

Here's the rake output:

$ rake jquery3
Running phantomjs /Users/tung/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/bundler/gems/opal-rspec-f9dd86052296/vendor/spec_runner.js "http://localhost:9999/"
Object freezing is not supported by Opal

..3.0.0

.................................................................................................................................................

Finished in 4.5 seconds (files took 0.372 seconds to load)
147 examples, 0 failures
$ 

[elia]

Thanks! I'm actually merging it 🙂
would be great if you can also change the Travis config to run it programmatically