Until 21st February 2016, the
matthieua version was licensed under the MIT license; you can see the commit which changes the license to GPL v3 here. The GPLv3 is of course still an open source license, although deliberately one with more invasive compliance conditions than the MIT license. It appears that the vendors of the product also make it available under a proprietary software license; purchasing such a license enables the purchaser to ignore the GPLv3 requirements.
graingert version was forked at (essentially) the last commit before the license change, so is still available under the MIT license; that the license has changed on later versions is irrelevant for continued use of the fork.
Am I safe using the fork in my commercial projects for free?
You can use either MIT or GPLv3 code in commercial projects without paying a fee to the copyright holders, as all open source licenses allow for commercial use. Both place some restrictions on what you do which are detailed in the respective licenses.