Add Ruby 2.4.1 to travis matrix

[timcraft]

No description provided.

[sdsykes]

Hmm, travis failed fetching external images during test. Not sure why that would be.

[timcraft]

@sdsykes Looks like a fakeweb issue. This works:

$ irb -Ilib -r fastimage
irb(main):001:0> FastImage.size(HTTPSImage)
=> [9545, 6623]

Whereas this doesn't:

$ irb -Ilib -r fakeweb -r fastimage
irb(main):001:0> FastImage.size(HTTPSImage)
=> nil

[sdsykes]

Yes, FakeWeb on 2.4.x is broken. But we are hoping for a new release very soon, see chrisk/fakeweb#57

In the mean time unless you want to propose using WebMock instead, I suggest we wait until FakeWeb is fixed before merging this change.

[timcraft]

I've used WebMock a lot and can definitely recommend it, but I think makes sense to wait for a new FakeWeb release in this case given you're already using that.

[sdsykes]

Eventually I made a release of a fork of FakeWeb chrisk/fakeweb#57 (comment) and added ruby 2.5 to the travis matrix.