npm: use npm_config_prefix instead of NPM_CONFIG_PREFIX

Recent versions of npm (at least 1.4.21) don't accept
NPM_CONFIG_PREFIX being valid. They expect the lower-case version
instead. Since I don't know for sure that older versions also accept
that, I keep both of them.
Merge pull request #100 from vincentbernat/fix/npmconfig

npm: use npm_config_prefix instead of NPM_CONFIG_PREFIX