The build instructions were outdated and I just updated them. Two things:
1. using esp8266 core 2.7.4 is fine as the firmware has been verified to compile with 2.7.4 (though 2.5.2 should also work fine)
2. for the UIPEthernet library, use the one that we’ve modified (it contains some dhcp fixes and customization). This is available in the OpenSprinkler github repository, and here is the direct download link:
https://github.com/OpenSprinkler/UIPEthernet/archive/fixes/dhcp.zip