compatibility with non-bash shells

[amireh]

• use . instead of source for sourcing the shell script
• fix a few deprecation-related notices
• export NVM_DIRto the environment with what we specify in nvm_root so that one can utilize it in scripts (if there was an nvm which command like rbenv's, this wouldn't have been necessary. I needed this to locate specific binaries)

[pcriv]

Thanks @amireh i'll try to merge this to master later today and release a new version to ansible galaxy

[amireh]

Thanks!