Unary negative literals should not be be reported as subexpressions

[jimweirich]

sexp = Ripper::SexpBuilder.parse("a + -1")
Sorcerer.subexpressions(sexp)

Currently returns

   ["a + -1", "a", "-1"]

It should return

   ["a + -1", "a"]

(same for -1.0 as well)