Enable composer cache

[goetas]

No description provided.

[goetas]

no big differences... maybe 10-15 % of improvements.

Is it worth?

[mattfarina]

So, I'll get a little technical here because this is interesting.

This is really a shift from crabbing checkouts of projects as a tarball and transferring it between GitHub and Travis CI to a setup where package versions as tarballs are stored locally in Travis (moved around as needed) and only new versions are fetched. The fact they are close in performance is a testament to the performance tuning done by GitHub.

Because GitHub has to regularly checkout a version and perform computation to create the tarballs I'm inclined to use the caching. I suspect caching is, on a combined system scale, lower on computation.

Anyone else have thoughts?

[mattfarina]

Also, this should be cherry picked to the 2.x branch once merged.

[goetas]

So you look "pro" to this feature, right?

[mattfarina]

@goetas Yeah, I'm pro-cache in this case. We can always undo this later pretty easily.

[goetas]

👍