Upgrade Hystrix to 1.5.18

[tinexw]

Hystrix 1.5.11 was re-released as 1.5.18 because this is the version that is internally used at Netflix and is considered more stable, s. Netflix/Hystrix#1891 and https://github.com/Netflix/Hystrix/releases/tag/v1.5.18

[codecov-io]

Codecov Report

Merging #3281 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3281      +/-   ##
============================================
+ Coverage     65.28%   65.35%   +0.07%     
- Complexity     1477     1481       +4     
============================================
  Files           188      188              
  Lines          6965     6965              
  Branches        847      847              
============================================
+ Hits           4547     4552       +5     
+ Misses         2104     2103       -1     
+ Partials        314      310       -4

Impacted Files	Coverage Δ	Complexity Δ
...oud/netflix/zuul/filters/post/SendErrorFilter.java	76.36% <0%> (+3.63%)	17% <0%> (+1%)	⬆️
...x/security/SecurityContextConcurrencyStrategy.java	77.77% <0%> (+16.66%)	8% <0%> (+3%)	⬆️

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[ryanjbaxter]

@tinexw would you mind submitting this against the 2.0.x branch? Then we can just merge it forward into master.

[tinexw]

@ryanjbaxter Sure

[tinexw]

I created a new PR (#3288) against the 2.0.x branch. Closing this one.