Support latest supported source version

[thyming]

Closes #90.

I investigated changing the build JDK for the processor to JDK 11, but there's a bug I'm hitting in the compiler that's preventing upgrade.

[codecov-io]

Codecov Report

Merging #92 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #92      +/-   ##
============================================
+ Coverage     90.18%   90.19%   +<.01%     
- Complexity      848      849       +1     
============================================
  Files            56       56              
  Lines          2925     2926       +1     
  Branches        135      135              
============================================
+ Hits           2638     2639       +1     
  Misses          226      226              
  Partials         61       61

Impacted Files	Coverage Δ	Complexity Δ
...java/org/derive4j/processor/DerivingProcessor.java	71.31% <100%> (+0.23%)	33 <1> (+1)	⬆️

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[jbgi]

Thanks! And sorry for late review, somehow missed github notif for your PR.
Will release soon with the changes.

[jbgi]

@thyming 1.1.1 should be on its way to maven central (built with jdk12, targeting 1.8). Thanks for PR!