For Pixi.js versions 3.0 and above default scaling can be set by changing PIXI.SCALE_MODES.DEFAULT:
PIXI.SCALE_MODES.DEFAULT = PIXI.SCALE_MODES.NEAREST;
For Pixi.js versions 1.5.0 and above the constants have been moved and scaling is now set as:
PIXI.scaleModes.DEFAULT = PIXI.scaleModes.NEAREST;