address remaining comments from #363
fix install from source on FreeBSD 10

Compile with CXX=c++ on FreeBSD, as FreeBSD >= 10.0 no longer has
GCC in the base system (c++ is hardlinked to clang++ on FreeBSD 10.0
and g++ on FreeBSD < 10).
remove duplicate install test

`test/slow/install` is identical to `test/slow/install from source`
except in the version installed.  Remove it.
use a more portable release in install tests

The tests for nvm install currently install v0.8.6, which doesn't
build on FreeBSD due to Linuxisms.  Switch to a more recent version
that does work.