Add v5.x to REBASE_ONTO dropdown

[bnoordhuis]

Build with Parameters has no option for v5.x in the REBASE_ONTO dropdown.

[Note to self: https://github.com/nodejs/node/pull/4231]

[rvagg]

@orangemocha or @joaocgreis can either of you take this one please?

[orangemocha]

done!