2015-07-17 io.js v2.4.0 Release

Notable changes

* src: Added a new `--track-heap-objects` flag to track heap object
allocations for heap snapshots (Bradley Meck)
https://github.com/nodejs/io.js/pull/2135.
* readline: Fixed a freeze that affected the repl if the keypress event
handler threw (Alex Kocharin) https://github.com/nodejs/io.js/pull/2107.
* npm: Upgraded to v2.13.0, release notes can be found in
https://github.com/npm/npm/releases/tag/v2.13.0 (Forrest L Norvell)
https://github.com/nodejs/io.js/pull/2152.

PR-URL: https://github.com/nodejs/io.js/pull/2189