2.5.5 - Critical bugfixes

This fixes 5 bugs, three of them critical and one serious, in the 2.5.x releases. Optiboot uploads were broken, attachInterrupt on the default settings was broken and would crash the sketch when an interrupt returned, the new Wire.endTransmission did not correctly implement the Wire API, resulting in "scanners" not working, and anything that checked for errors thinking that actual error conditions were successful any any successful transaction had failed. There was also another problem with the buffer size in Wire that broke some libraries in the wild, most notably the Adafruit OLED library.

These should all be resolved now.