Make Julia 1.6 the minimum version required for this package

[isaamael]

Hi ！ erro as follow:

julia version 1.4.0
R version 4.3.3

Session Info

> packageVersion("JuliaCall")
[1] ‘0.16.6’
> julia_setup()
Julia version 1.4.0 at location /public/home/xuruiqiang/software/julia/julia1.4.0/julia-1.4.0/bin will be used.
Loading setup script for JuliaCall...
WARNING: replacing module JuliaCall.
ERROR: LoadError: Preferences.jl can only be used on Julia v1.6+!
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] top-level scope at /public/home/xuruiqiang/.julia/packages/Preferences/enLcN/src/Preferences.jl:4
 [3] include(::Module, ::String) at ./Base.jl:377
 [4] top-level scope at none:2
 [5] eval at ./boot.jl:331 [inlined]
 [6] eval(::Expr) at ./client.jl:449
 [7] top-level scope at ./none:3
in expression starting at /public/home/xuruiqiang/.julia/packages/Preferences/enLcN/src/Preferences.jl:3
ERROR: LoadError: Failed to precompile Preferences [21216c6a-2e73-6563-6e65-726566657250] to /public/home/xuruiqiang/.julia/compiled/v1.4/Preferences/pWSk8_Znzu6.ji.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] compilecache(::Base.PkgId, ::String) at ./loading.jl:1272
 [3] _require(::Base.PkgId) at ./loading.jl:1029
 [4] require(::Base.PkgId) at ./loading.jl:927
 [5] require(::Module, ::Symbol) at ./loading.jl:922
 [6] include(::Module, ::String) at ./Base.jl:377
 [7] top-level scope at none:2
 [8] eval at ./boot.jl:331 [inlined]
 [9] eval(::Expr) at ./client.jl:449
 [10] top-level scope at ./none:3
in expression starting at /public/home/xuruiqiang/.julia/packages/RCall/dDAVd/src/RCall.jl:3
LoadError("/public/home/xuruiqiang/R_libs/JuliaCall/julia/setup.jl", 72, ErrorException("Failed to precompile RCall [6f49c342-dc21-5d91-9882-a32aef131414] to /public/home/xuruiqiang/.julia/compiled/v1.4/RCall/8GFyb_xDoml.ji.")) Error in .julia$cmd(paste0("Base.include(Main,\"", system.file("julia/setup.jl",  : 
  Error happens when you try to execute command Base.include(Main,"/public/home/xuruiqiang/R_libs/JuliaCall/julia/setup.jl") in Julia.
                        To have more helpful error messages,
                        you could considering running the command in Julia directly

[ViralBShah]

I think we should make 1.6 the min supported Julia version. Perhaps sufficient to add it to the README.