That is not anti-aliasing but the GL_LINEAR filter which is set by default in OpenGL. There are other types of filters such as GL_NEAREST (the pixelated look).
You can specify which filter to use in Slick2D when loading an image. See the last parameter on the "new Image()" method in the JavaDoc here
Slick2D has two filter options available Image.FILTER_LINEAR (the default) and Image.FILTER_NEAREST which you can set when loading the Image.
Image myImage = new Image("myimage.png", false, Image.FILTER_NEAREST);