yarn: default to an older nodejs on < 10.8

[mojca (Mojca Miklavec)]

nodejs6 only seems to work on 10.8 and later (see #53165 and many other tickets).

Based on comments in the portfiles it seems that nodejs4 and 5 require at least 10.6 and nodejs requires at least 10.7. I suspect that the build failure on 10.7 might be only a matter using a too old compiler on 10.7 though and the following blacklist might not be sufficient:

compiler.blacklist *gcc* {clang < 300}

It would be nice to fallback to an older version of nodejs (perhaps version 5) on < 10.8 (or on < 10.7 once the issue of building nodejs6 gets resolved on Lion).

Btw: is the isomarcte's email in Yarn misspelled?