This commit adds mltonHEAD.
Summary of all the changes in this branch:
* mlton20130715
- for this one I just copy and pasted the old default.nix
- I have tested some projects that compile with this version and don't
compile with the newer version, so I think it makes sense to keep
the old version
* mlton20180207Binary
- This is used to build the source derivations
* mlton20180207
- latest release
* mltonHEAD
- latest commit to the master branch of the github repo at the time of
creating this derivation
bb7ed2cd81