Bump the cache save timeout. (#286)
Bump the cache save timeout. (#286)

Previously one of the runs was failing to save its cache at the end; for
example:
https://travis-ci.org/google/proto-lens/jobs/468129199
with the error:
```
running `casher push` took longer than 180 seconds and has been aborted.
You can extend the timeout with `cache.timeout`. See https://docs.travis-ci.com/user/caching/#Setting-the-timeout for details
```

The default limit is 3 minutes.  When I increased the limit, I saw the cache
save took 214 seconds.  This change bumps the limit to 10 minutes to be safe
in case of, e.g., network load.
google/proto-lens