Add Java 11 executor

[Xyene]

No automated tests for this currently. If we're lucky and bionic works, we'll gain Java 9 coverage at most.

[codecov-io]

Codecov Report

Merging #526 into master will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
- Coverage   72.98%   72.83%   -0.15%     
==========================================
  Files         126      127       +1     
  Lines        3949     3980      +31     
==========================================
+ Hits         2882     2899      +17     
- Misses       1067     1081      +14

Impacted Files	Coverage Δ
dmoj/executors/JAVA11.py	0% <0%> (ø)
dmoj/cptbox/tracer.py	82.67% <0%> (-0.5%)	⬇️
dmoj/executors/base_executor.py	77.98% <0%> (-0.46%)	⬇️
dmoj/checkers/floats.py	85.93% <0%> (-0.11%)	⬇️
dmoj/executors/clang_executor.py	100% <0%> (ø)	⬆️

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