Fix tests for 1.8.7

Some of the tests had 1.9.x syntax in them. Fixed.
Bring JRuby features up to par with tests.

JRuby's ImageIO doesn't support PPM, so disable PPM tests for JRuby.
Remove :exact initializer option.

Was never part of a released version, and needs to be rethought.
Only expose x + y scaling functions.

This makes the library responsible for handling the details.