Phoenix 1.4 Upgrade

[iloveitaly]

Thanks for the amazing work on this buildpack! I ran into a couple issues when setting this up with recent phoenix project:

• Use --sname to allow iex console sessions to connect to the running process
• Clear out node_modules, regardless of node/npm version, when clean_cache is enabled
• Use cp -R to avoid the build failing if a node module is symlinked. The latest phoenix package.json symlinks by default.