This topic got answered in another thread, but I’ll post the answer here since it’s important:
After extensive testing with 8 of the latest versions of the IDE, I can confirm that the EnviroDIY board configuration file that is currently posted on Github will work with Arduino IDE versions 1.6.5 through 1.6.12.
There was an update done to the IDE starting with version 1.6.11 that caused an incompatibility with the Mayfly configuration file on GitHub. But I just updated that config file and tested it with version 1.6.11 and 1.6.12, and everything is working fine now. Thanks for pointing out the issue, Paul and Marion.