Travis caching

Setup instructions

Notes

One problem is that storing the cache is counted as part of the build. That adds something like 4 mins; without that the build would be faster still. It's an open issue on travis - they will eventually change so that it's not. Upload of the cache anyway does not affect the build status.

The cache is likely to cause confusion sometimes. E.g. the env gets messed up somehow. You can delete the cache from travis. You probably already know this.

I have not fully understood the rules for keying the cache. The test environment is supposed to use a cache, while the packaging environment is not (it wouldn't make much difference there - the packaging process creates a new temporary env anyway). PRs etc should inherit "a copy of" the master cache (i.e. not update it - but they then get their own cache).