Fixed built-in Master I²C driver for Arduino ESP8266 core. Sorry it doesn't support slave I²C mode.
Click here to download the ZIP file
Copy and replace "twi.h", "core_esp8266_si2c.c" in folder %USERPROFILE%\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0\cores\esp8266
Copy and replace "Wire.h", "Wire.cpp" in folder %USERPROFILE%\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0\libraries\Wire
Enjoy.