julia compatibility

[korbinian90]

the Julia compatibility in the compat section is set to
julia = "1"

When trying to use the package with Julia 1.0.5, you can add the package, but get a bunch of (not helpful) errors.
I think only the oldest Julia version that is automatically tested, should be supported, so
julia = "1.4"
Otherwise, changes can break things for all users with julia <1.4 without noticing.

With setting the compatibility to the tested version, people with older Julia versions just get older versions of this package that are tested for their old Julia version, instead of error messages.

[Tokazama]

Sorry. I thought this would be been handled by the package system since I dropped those as a result of PrettyTables.jl dropping support for those versions.

Edit: looks like PrettyTables.jl didn't set this in the compat. I'll look closer and see if I really need to drop support for those versions.