adapt install.sh for npm 2

npm 2 no longer has an engines field in its package.json, so don't check for that. This also incorporates a bunch of cleanups suggested by `shellcheck`, and drops the polite fiction that running Node < 0.8 is going to give you an acceptable experience in 2014 with npm. It makes up for this by adding 0.8 to the list of versions that gets the latest npm by default.

@isaacs should probably review this to ensure I didn't do anything boneheaded.

@rockbot, I will also copy this to a separate PR on newww.
This makes the script only runnable in Bash.  sh doesn't handle `$()`