eliminate NEON_NODE_ABI environment variable

[dherman]

From @dherman on January 9, 2017 20:57

With #109 we extract the node modules ABI version number from the headers instead of pulling it from the environment variable. When we upgrade to neon 0.2 we should get rid of the environment variable entirely.

(This was done in #37 but rolled back in #41 to wait until we're ready for 0.2.)

Copied from original issue: neon-bindings/neon-cli#43