Avoid throwing depwarn for zeros/ones methods defined in Base

[timholy]

These improve upon the deprecations added in #12, which are inadvertently triggered even for syntax classically supported by Base:

julia> zeros(1:5)
WARNING: zeros(inds::UnitRange...) is deprecated, use fill!(OffsetArray{Float64}(inds),0) instead.
 in depwarn(::String, ::Symbol) at deprecated.jl:64
 in zeros(::UnitRange{Int64}) at deprecated.jl:50
 in eval(::Module, ::Any) at boot.jl:234
 in eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:64
 in macro expansion at REPL.jl:95 [inlined]
 in (::Base.REPL.##3#4{Base.REPL.REPLBackend})() at event.jl:68
while loading no file, in expression starting on line 0
OffsetArrays.OffsetArray{Float64,1,Array{Float64,1}} with indices 1:5:
 0.0
 0.0
 0.0
 0.0
 0.0

[timholy]

I should clarify that on this branch:

julia> zeros(1:5)
5-element Array{Int64,1}:
 0
 0
 0
 0
 0

julia> zeros(1:5, -2:2)
WARNING: zeros(ind1::UnitRange,ind2::UnitRange,inds::UnitRange...) is deprecated, use fill!(OffsetArray{Float64}((ind1,ind2,inds...)),0) instead.
 in depwarn(::String, ::Symbol) at deprecated.jl:64
 in zeros(::UnitRange{Int64}, ::UnitRange{Int64}) at deprecated.jl:50
 in eval(::Module, ::Any) at boot.jl:234
 in eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:64
 in macro expansion at REPL.jl:95 [inlined]
 in (::Base.REPL.##3#4{Base.REPL.REPLBackend})() at event.jl:68
while loading no file, in expression starting on line 0
OffsetArrays.OffsetArray{Float64,2,Array{Float64,2}} with indices 1:5×-2:2:
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0

[alsam]

Thanks for the pull requests!