Latest version v0.6 has wrong URL_NIST in MakeFile

[bremme]

First of all, thanks a lot for all the hard work to make Refprop work on Linux. Perhaps the master branch is just fine for this project, but I usually just check out the most recent version. So I checked out v0.6, I noticed that the URL_NIST variable in the MakeFile is wrong. Since you're also advising people to download the latest release you could perhaps make a note of this or push a new version. Thanks.

[jowr]

Good point, we really do have to release an up-to-date version.