Use this Guide from Stack Exchange to export from Blender 2.79:
For Blender 2.79(x), use the Khronos Blender Exporter:
To install it, copy the
scripts/addons/io_scene_gltf2 folder to your
folder. Then, re-launch Blender, go into User Preferences ->
Addons, search for glTF, and enable the export plugin there. Please do
read the plugin’s documentation to learn how to author PBR materials in
Blender that will survive the export.