You can always get the latest stable release (currently v1.0.20171215) here
   https://github.com/rordenlab/dcm2niix/releases
A version is also distributed with MRIcroGL, but that version of dcm2niix to lag behind the latest stable release because as MRIcroGL itself is mature and there is little need by itself to develop regular releases. You can get the latest stable MRIcroGL release here
  https://github.com/neurolabusc/MRIcroGL/releases
The latest Linux and MacOS versions of MRIcroGL include the feature "Check for updates" in the preferences window that will determine if your copy of dcm2niix is up to date.

As an aside, user of older versions of Linux my find that the version of dcm2niix from the Github webpage may not work on their computer while the one distributed with MRIcroGL does. The github release is generated on a modern Linux system and will require more recent libraries. For those who like old editions of Linux, or anyone who wants to build the latest build of dcm2niix you can always run:

git clone https://github.com/rordenlab/dcm2niix.gi...
cd dcm2niix
mkdir build && cd build
cmake ..
make

This will give you the very latest build, and is not necessarily a stable release.