Updates to travis.yml?

[tom95]

Metacello's .travis.yml may need an update, for one the Pharo versions being built might benefit from a more recent list.

Further, in particular for Squeak, caching the folder containing the image is probably not the right way. According to @fniephaus the image that is cached and the image that is already provided by smalltalkCI should for the most part be identical, as such downloading the cache is a likely unnecessary overhead and always prevents updates to the pre-built smalltalkCI from arriving in Metacello builds.

[dalehenrich]

GemStone should have the travis cache turned on and I think that there is a way to make it conditional for GemStone only ... other than that submit a PR with your proposed changes ...

[dalehenrich]

@tom95 ... take a look at the smalltalkCI .travis.yml file for hints on making the cache conditional on GemStone ... I am not a yaml expert, so it might take some experimentation to get it working, if you want to take a crack at it:)