Add Node v0.6 back to the .travis.yml list.
Add Node v0.6 back to the .travis.yml list.

This is a follow-up to c502fe451ab07e6fde9ef568b762fd9a481d5e72, where I
moved 0.6 to the allow_failures section but mistakenly removed it from the
list of engines. Hopefully this fix gets the tests running at all again.

jamestalmage/ast-types,benjamn/ast-types,benjamn/ast-types,gabelevi/ast-types,benjamn/ast-types,cpojer/ast-types,marcioj/ast-types