Migrate to NodeJS 9

Since few days, the Linux Duniter Desktop version no more succeed to compute blocks.

It seems to be linked with NodeJS 8, because I successfully build versions using NodeJS 9: https://forum.duniter.org/t/testeurs-g1-test-nouvelle-pre-release-a-tester/4074/134

So, I suggest to migrate immediately to NodeJS 9. This was planned anyway for #1262 (closed).

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