Implement LRU cache for platforms

[cpetrov]

As @ralfstx suggested, to prevent platforms from piling up in ~/.tabris-cli/platforms, least recently used platforms should be removed.

For a reasonable size of the platforms cache, last recently used 10 platforms could be kept in cache.

[cpetrov]

Currently, cache pruning is only applied to nightly platforms, since they are most likely to pile up. The cache keeps a single nightly platform per OS.