Using the cache with Pharo64-7.0 builds crashes the build

[jbrichau]

See https://travis-ci.org/SeasideSt/Grease/jobs/489242759
The build for Pharo64-7.0 crashes when there is already a cached download present.
So, when using this in your config:

cache:
  directories:
    - $SMALLTALK_CI_CACHE

It crashes as follows:

Downloading Pharo64-7.0 image...
Downloading the latest 70 Image:
    http://files.pharo.org/get-files/70/pharo64.zip
mv: overwrite ‘Pharo7.0-32bit-b882595.sources’? 
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

[fniephaus]

I believe the problem is that smalltalkCI does not check for the files it needs for the build, it should not start downloading an image if in cache. I don't have time to maintain all of the Pharo logic in smalltalkCI, but it should be relatively easy to fix. Contributions are more than welcome!