Check if memcached is running during memcache spec, and fail if not running

[technicalpickles]

I was trying to run the test suite locally, and it failed here because I didn't have memcached running (yet):

  1) Rack::MiniProfiler::MemcacheStore page struct storage can store a PageStruct and retrieve it
     Failure/Error: @client.set("#{@prefix}#{page_struct[:id]}", Marshal::dump(page_struct), @expires_in_seconds)

     Dalli::RingError:
       No server available
     # ./lib/mini_profiler/storage/memcache_store.rb:22:in `save'
     # ./spec/lib/storage/memcache_store_spec.rb:17:in `block (4 levels) in <top (required)>'

This adds a alive? method that the spec can check, and give more specific information to the user. I've also included a docker-compose.yml to give a one-liner to start memcached.