Fix "~/.nvm/nvm.sh: No such file ..." error on travis

[zetavg]

It seems like nvm is installed in the wrong path ('/usr/local/opt/nvm') in some of the builds. This PR moves

export NVM_DIR=~/.nvm

to set the correct NVM directory before installing

which nvm || curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.29.0/install.sh | bash

to fix this.

Ref: https://github.com/creationix/nvm#install-script

[bleonard]

Thanks!