fix(limit): invalidate oldest cache when reaching limit size

[zhenwenc]

When the number of cached requests reaches the configured limit, the oldest cached item should be removed from the store.

#247

[codecov[bot]]

Codecov Report

Merging #248 (4d16e29) into master (2d51cee) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #248   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          235       238    +3     
=========================================
+ Hits           235       238    +3     

Impacted Files	Coverage Δ
src/memory.js	100.00% <100.00%> (ø)
src/redis.js	100.00% <100.00%> (ø)
src/serialize.js	100.00% <0.00%> (ø)

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[cowboy-bebug]

cc @RasCarlito