runtime: Bump ring to 0.16.11, snow to 0.6.2

[kostko]

Fixes #2666

Also bumps Rust nightly to 2020-02-16.

[codecov]

Codecov Report

Merging #2685 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2685      +/-   ##
==========================================
+ Coverage   63.11%   63.16%   +0.04%     
==========================================
  Files         376      376              
  Lines       35319    35319              
==========================================
+ Hits        22292    22309      +17     
+ Misses      10226    10211      -15     
+ Partials     2801     2799       -2     

Impacted Files	Coverage Δ
...ompute/txnscheduler/algorithm/batching/batching.go	78.66% <0.00%> (-6.67%)	⬇️
go/storage/metrics.go	72.64% <0.00%> (-5.13%)	⬇️
go/storage/api/root_cache.go	70.11% <0.00%> (-2.30%)	⬇️
go/storage/mkvs/urkel/lookup.go	72.72% <0.00%> (-2.28%)	⬇️
go/storage/database/database.go	74.56% <0.00%> (-1.76%)	⬇️
go/worker/compute/executor/committee/node.go	62.00% <0.00%> (-1.38%)	⬇️
go/worker/storage/service_external.go	51.61% <0.00%> (ø)	⬆️
go/worker/compute/txnscheduler/committee/node.go	64.50% <0.00%> (+0.30%)	⬆️
go/worker/registration/worker.go	67.15% <0.00%> (+0.49%)	⬆️
go/storage/api/grpc.go	70.56% <0.00%> (+0.66%)	⬆️
... and 10 more

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