build: re-enable V8 snapshots

Snapshots had been previously disabled because of a security
vunerability. This has been fixed (ref:
https://github.com/iojs/io.js/issues/1631#issuecomment-100101375)

Also, re-enable snapshots for ARMv6 builds. There were previous build
issues that have been fixed.

Fixes: https://github.com/iojs/io.js/issues/1631
PR-URL: https://github.com/iojs/io.js/pull/1663


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>