signed(UInt8) is not defined until Julia 1.5

[andyferris]

We should probably either move up the project dependency to Julia 1.5 or else workaround the issue that is making all the CI fail.

https://github.com/JuliaData/Avro.jl/blob/main/src/types/binary.jl#L151

   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.3.1 (2019-12-30)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> signed(UInt8)
ERROR: MethodError: Cannot `convert` an object of type Type{UInt8} to an object of type Signed
Closest candidates are:
  convert(::Type{T}, ::T) where T<:Number at number.jl:6
  convert(::Type{T}, ::Number) where T<:Number at number.jl:7
  convert(::Type{T}, ::Ptr) where T<:Integer at pointer.jl:23
  ...
Stacktrace:
 [1] signed(::Type) at ./int.jl:170
 [2] top-level scope at REPL[1]:1