2015-04-06 io.js v1.6.4 Release

Notable changes:

* npm: upgrade npm to 2.7.5. See the npm CHANGELOG.md for details.
Includes two important security fixes.
https://github.com/npm/npm/blob/master/CHANGELOG.md#v275-2015-03-26
* openssl: preliminary work has been done for an upcoming upgrade to
OpenSSL 1.0.2a #1325 (Shigeki Ohtsu). See #589 for additional details.
* timers: a minor memory leak when timers are unreferenced was fixed,
alongside some related timers issues #1330 (Fedor Indutny). This
appears to have fixed the remaining leak reported in #1075.
* android: it is now possible to compile io.js for Android and related
devices #1307 (Giovanny Andres Gongora Granada).