Singular.jl 0.5.4 released

Today Singular.jl 0.5.4 has been released.

Diff since v0.5.3

Closed issues:

• things to re-enable once n_unknown is working again (#61)
• Iterators need to be implemented (#433)

Merged pull requests:

• enable dimension over rings (ZZ) (#435) (@tthsqe12)
• Access caches using get! / AbstractAlgebra.get_cached! (#436) (@fingolfin)
• Move base_ring to start of our structs (#437) (@fingolfin)
• Verify some type parameters in constructors (#438) (@fingolfin)
• bump AA and Nemo versions (#439) (@tthsqe12)