Infinite recursion on nightly

[vchuravy]

From https://travis-ci.org/JuliaParallel/DistributedArrays.jl/jobs/525992108

test construct Array from (Sub)DArray: Error During Test at /home/travis/build/JuliaParallel/DistributedArrays.jl/test/darray.jl:216
  Test threw exception
  Expression: vec(S5) == D[1, 1:4]
  StackOverflowError:
  Stacktrace:
   [1] ==(::SubArray{Float64,1,DArray{Float64,2,Array{Float64,2}},Tuple{Int64,UnitRange{Int64}},false}, ::Array{Float64,1}) at /home/travis/build/JuliaParallel/DistributedArrays.jl/src/darray.jl:430
   [2] ==(::SubArray{Float64,1,DArray{Float64,2,Array{Float64,2}},Tuple{Int64,UnitRange{Int64}},false}, ::Array{Float64,1}) at /home/travis/build/JuliaParallel/DistributedArrays.jl/src/darray.jl:431 (repeats 57993 times)
   [3] ==(::Array{Float64,1}, ::SubArray{Float64,1,DArray{Float64,2,Array{Float64,2}},Tuple{Int64,UnitRange{Int64}},false}) at /home/travis/build/JuliaParallel/DistributedArrays.jl/src/darray.jl:436
   [4] eval_test(::Expr, ::Expr, ::LineNumberNode, ::Bool) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.3/Test/src/Test.jl:244
   [5] top-level scope at /home/travis/build/JuliaParallel/DistributedArrays.jl/test/darray.jl:216
   [6] top-level scope at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.3/Test/src/Test.jl:1113
   [7] top-level scope at /home/travis/build/JuliaParallel/DistributedArrays.jl/test/darray.jl:186
   [8] top-level scope at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.3/Test/src/Test.jl:1113
   [9] top-level scope at /home/travis/build/JuliaParallel/DistributedArrays.jl/test/darray.jl:183

[JeffBezanson]

Caused I believe by JuliaLang/julia#30889. We can work around it by defining copy.