Polymake.jl 0.6.1 released

Today Polymake.jl 0.6.1 has been released.

Diff since v0.6.0

Merged pull requests:

• Serialization: Allow saving of graphs (#367) (@lkastner)
• tests: make sure to run 4ti2 and not fall back to other rules (#368) (@benlorenz)
• Bump version to 0.6.1 (#371) (@benlorenz)