Always update nvm
Always update nvm

If we do not know what version of `nvm` which `travis-build` web app
is serving at the build script compile time (with some constant known
to this file), we must fetch `nvm.sh` from the web app at run time to
find it.
If we *are* fetching `nvm.sh` for any reason, though, we might as
well use it to update nvm.sh.

craigcitro/travis-build,JuliaCI/travis-build,craigcitro/travis-build,kevmoo/travis-build,JuliaCI/travis-build,andyli/travis-build,kevmoo/travis-build,andyli/travis-build