Added backward compatibility with Julia 0.6

[mdashti]

There were some related discussions here: https://github.com/MikeInnes/MacroTools.jl/commit/18448ba5e26624345f6f4eef9f05e4aa09b03387#r30081444

This one-liner fixes backward compatibility with 0.6. All the 0.6 packages that depend on MacroTools and are marked as __precompile__(true) won't work without this.