Polymake.jl 0.6.0 released

Today Polymake.jl 0.6.0 has been released.

Diff since v0.5.8

Merged pull requests:

• improve and unify save+load, add access to shell variables (#363) (@benlorenz)
• Add tests for constructing BigObject from int and graph properties (#364) (@lkastner)
• Fix typemapping for Array{String} (#365) (@alexej-jordan)
• bump to 0.6, drop julia < 1.6 (#366) (@benlorenz)