The AtTiny85 board is powered via its USB, so 5V.
The HMC5883L CJ-M49 board has a voltage regulator so it is spec'd to run 3.3V or 5V.
In fact the Uno can access it directly with Wire.h.
The AtTiny85 fails (I tried with or without pullups). The bitBang Wire library for the AtTiny/Digistump
does not clearly code how it handles the CPU frequency so I want to make sure it's known to work at 16 or 16.5MHz with F_CPU matching.
Sadly I just destroyed my ADXL345 while testing 
I wired the power backwards and I think the voltage regulator is fried (it was super hot and I noticed the typical smell) 
It looks like this stuff:
Pins are Gnd, 3.3V (not wired), 5V, SCL, SDA, DRDY (not wired)
On an Uno it's simple:
On AtTiny85, this shows the pullups, similar setup to what I use for my I2C with PB0 and PB2 (except I use adjustable resistors, 4.7kOhm)