1-liner bugfix for Interval Set

[Guttz]

I'm pretty sure this is a bug. Checked the python implementation and it looked similar. None of them have tests though.

This change is necessary or else the last else if will never be executed and the function returns wrong results for some cases. For example, the following tests will fail without this change, even though they are correct:

TestRemoveRangeSameStart() {
	i := antlr.NewIntervalSet()
	i.addInterval(antlr.NewInterval(1, 10))
	i.removeRange(antlr.NewInterval(1, 7))

	suite.Len(i.intervals, 1)
	suite.Equal(7, i.intervals[0].Start)
	suite.Equal(10, i.intervals[0].Stop)
}

TestRemoveRangeOverStartBound() {
	i := antlr.NewIntervalSet()
	i.addInterval(antlr.NewInterval(5, 15))

	i.removeRange(antlr.NewInterval(1, 10))
	suite.Len(i.intervals, 1)
	suite.Equal(10, i.intervals[0].Start)
	suite.Equal(15, i.intervals[0].Stop)
}