disable d098d82 for shortcut operator temporarily

[mame]

d098d82 introduces a stack inconsistency when it is a shortcut operator.

a = { :foo => { :bar => 0 } }
p((a[:foo] ||= {})[:bar] ||= 42)
#=> expected: 0, actual: nil

Note that this is just a temporary fix to disable d098d82; a in a[b] ||= c is evaluated twice.

[matz]

29d3a40 causes TravisCI fail. need to fix.