doc: update OS level support for AIX
Update OS level supprot for AIX. Based on discussions with the AIX team we've have decided that the minimum support level should be 7.1. This is because support for AIX 6.1 will likely end during the lifespan of the upcoming Node version 8 release.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows [commit guidelines][]
Affected core subsystem(s)
doc