@windows_only is deprecated in Julia v0.5.0

[aus70]

WARNING: `@windows_only` is deprecated, use `@static if is_windows()` instead

I guess that's easy to fix.

[ivan-krukov]

Also getting the following during compilation:

running: /Applications/Julia-0.5.app/Contents/Resources/julia/bin/julia /Users/xxx/.julia/v0.5/BuildExecutable/src/build_sysimg.jl /Applications/Julia-0.5.app/Contents/Resources/julia/lib/libwf native /var/folders/rk/xv9h1cj162vctr73yq2g18dw0000gn/T/tmplR2TZm/userimg.jl --force
WARNING: `@unix_only` is deprecated, use `@static if is_unix()` instead
 in depwarn(::String, ::Symbol) at ./deprecated.jl:64
 in depwarn(::String, ::Symbol) at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
 in @unix_only(::Any) at ./deprecated.jl:492
 in include_from_node1(::String) at ./loading.jl:488
 in include_from_node1(::String) at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
 in process_options(::Base.JLOptions) at ./client.jl:262
 in _start() at ./client.jl:318
 in _start() at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
while loading /Users/xxx/.julia/v0.5/BuildExecutable/src/build_sysimg.jl, in expression starting on line 8

[JSybrandt]

Bump, still happening.