raise prechecker nodejs/npm to stable when possible

This is a followup of MDLSITE-5134, where we changed from os-global nodejs & npm binaries to dynamic nvm-managed ones.

The prechecker job was left using v6 nodejs (v3 npm) because the job still runs branches <= 31 that are not ready for nodejs v8 (npm v5) (see MDL-59094).

So this issues is about to raise prechecker nodejs/npm versions to "stable" once it's guaranteed that there aren't <= 31 jobs executed.

The alternative to this is to make it programmatically (within the checker, discern based in the branch), but looks like an unnecessary hack.

Also it may be possible to backport MDL-59094 to 31_STABLE (now in security-only support), so we can apply for nodejs v8 once guaranteed there aren't <= 30, instead of <= 31.

pinging poltawski (backporting?) and mudrd8mz (plugins?) about this.

Ciao