`_mean` is ambigous

[vchuravy]

julia> mean(a)
ERROR: MethodError: Statistics._mean(::DArray{Float64,2,Array{Float64,2}}, ::Colon) is ambiguous. Candidates:
  _mean(A::DArray, region) in DistributedArrays at /home/vchuravy/.julia/dev/DistributedArrays/src/mapreduce.jl:139
  _mean(A::AbstractArray, ::Colon) in Statistics at /build/julia/src/julia/usr/share/julia/stdlib/v1.0/Statistics/src/Statistics.jl:131
Possible fix, define
  _mean(::DArray, ::Colon)
Stacktrace:
 [1] #mean#1(::Function, ::Function, ::DArray{Float64,2,Array{Float64,2}}) at /build/julia/src/julia/usr/share/julia/stdlib/v1.0/Statistics/src/Statistics.jl:128
 [2] mean(::DArray{Float64,2,Array{Float64,2}}) at /build/julia/src/julia/usr/share/julia/stdlib/v1.0/Statistics/src/Statistics.jl:128
 [3] top-level scope at none:0

[ViralBShah]

Fixed in #218