Updates for Flux 0.15

[DrChainsaw]

Should maybe tag this as breaking since recurrent layers are quite different now...

[codecov]

Codecov Report

Attention: Patch coverage is 81.95489% with 48 lines in your changes missing coverage. Please review.

Project coverage is 92.94%. Comparing base (4a3f464) to head (9034ab5).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/serialize/traceprobes.jl	76.54%	19 Missing ⚠️
src/serialize/serialize.jl	77.77%	10 Missing ⚠️
src/serialize/namingutil.jl	74.28%	9 Missing ⚠️
src/deserialize/ops.jl	88.52%	7 Missing ⚠️
src/shapes.jl	88.88%	2 Missing ⚠️
src/validate.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   96.18%   92.94%   -3.24%     
==========================================
  Files          15       16       +1     
  Lines        1152     1319     +167     
==========================================
+ Hits         1108     1226     +118     
- Misses         44       93      +49     

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