Make helios-client guava 20 compatible.

[daniel-kristjansson]

This changes some guava transform calls to transformAsync so we're compatible with guava 20.

We still pull in guava 19 for now.

[davidxia]

why didn't tests run?

[davidxia]

Tests don't seem to run for forked branches.

[mattnworb]

@davidxia there is a setting in CircleCI for only building pull requests that come from the same repo, which makes sense security-wise in case your CircleCI build has any sensitive info in it like encrypted env vars, etc. I don't think we have any of those so the setting could probably be disabled.

[davidxia]

@mattnworb Thanks. I've changed that.

[davidxia]

@rohansingh Just curious. Any idea why we don't use AsyncFunction in HelisoClient in places like https://github.com/spotify/helios/blob/master/helios-client/src/main/java/com/spotify/helios/client/HeliosClient.java#L273?

[mattnworb]

@daniel-kristjansson thanks!

[mattnworb]

@davidxia AsyncFunction is a function that returns a ListenableFuture<T>. If you don't have a future to return, then you just use Function. It is basically the same difference between thenApply and thenCompose with CompletableFutures.