[build] Allow failure of the JDK9 build in Travis CI

[simonbasle]

No description provided.

[reactorbot]

Fast-track requested by @simonbasle

[codecov-io]

Codecov Report

Merging #924 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #924      +/-   ##
============================================
+ Coverage     83.51%   83.52%   +0.01%     
- Complexity     3313     3315       +2     
============================================
  Files           323      323              
  Lines         26226    26226              
  Branches       4861     4861              
============================================
+ Hits          21902    21905       +3     
  Misses         2865     2865              
+ Partials       1459     1456       -3

Impacted Files	Coverage Δ	Complexity Δ
.../java/reactor/core/publisher/FluxLimitRequest.java	94% <0%> (-4%)	5% <0%> (ø)
...rc/main/java/reactor/core/publisher/FluxRange.java	83.33% <0%> (-1.34%)	5% <0%> (ø)
...ava/reactor/core/publisher/WorkQueueProcessor.java	70.76% <0%> (-0.85%)	19% <0%> (ø)
...ava/reactor/core/publisher/EventLoopProcessor.java	78.14% <0%> (ø)	49% <0%> (ø)	⬇️
...rc/main/java/reactor/core/publisher/Operators.java	72.15% <0%> (+0.18%)	84% <0%> (ø)	⬇️
...java/reactor/core/publisher/FluxBufferTimeout.java	74.6% <0%> (+0.79%)	2% <0%> (ø)	⬇️
...n/java/reactor/core/publisher/DirectProcessor.java	86.66% <0%> (+0.83%)	30% <0%> (+1%)	⬆️
...eactor/core/publisher/ParallelMergeSequential.java	81.34% <0%> (+1.03%)	7% <0%> (ø)	⬇️
...va/reactor/core/publisher/ParallelMergeReduce.java	71.9% <0%> (+1.65%)	3% <0%> (ø)	⬇️
...c/main/java/reactor/core/scheduler/WorkerTask.java	83.01% <0%> (+3.77%)	19% <0%> (+1%)	⬆️

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