Merge pull request #68 from theuni/cache

fix: use don't use methods that require new ruby version
Fix issue #63: use lxc-execute instead of lxc-start
Merge pull request #69 from gurnec/lxc-execute

Fix issue #63: use lxc-execute instead of lxc-start
Add warning in release notes about lxc-execute
Default back to lxc-start, with option for lxc-execute
cache: require the enable_cache key to be set to true in order to use cache functionality
update docs for enable_cache option