Neil, for the develop branch I would point to a specific commit. The current commit in develop tonight is located here:
For my platformio.ini file’s lib deps, I would point to with the syntax:
The latest release that PlatformIO can recognize is v0.12.2, which refers to the Master branch. I see that your 0.15.3 is the current Develop branch, but that won’t be recognizable by PlatformIO until it is merged to Master.