tla+ toolbox: 1.5.7 -> 1.6.0

Bumps to new release of TLA+ Toolbox.

Main change:
- toolbox now includes JRE so those external dependencies are no
longer needed.

Testing:
- nix-build -A tlaplusToolbox
- ./result/bin/tla-toolbox
- pdf generation
- validation of a simple pluscal model