Update and Clean documentation about new "Ubuntu-64bit / OpenVZ / Bluebox" stack

Since Travis moved most workers to Ubuntu 12.04/64-bit on top of OpenVZ, the current documentation is quite outdated. Since users are complaining either via google group, irc channel or github issues, I file this issue to track the documentation improvements we need to bring up as soon as possible.

### Goals:

 * Remove stuff around Virtualbox and Vagrant (where no more applicable)
 * Replace 32-bit by 64-bit (or just avoid mention it?)
 * DRY it?
 * Give some details about OpenVZ/bluebox that matter for the end-users
   * Why not refer to known issues like https://github.com/travis-ci/travis-ci/issues/932
 * During the "migration/upgrade phase", maybe give some kind of "Operating System | language" mapping (ruby => 32bit, python => 64bit, etc)
 * Maybe drop a word about upcoming Operating System selection feature ?
 * List the known issues (and possible workarounds) around "32 bit vs 64 bit" requirements, like:
   * [Android SDK needs a 32bit-capable worker](https://github.com/travis-ci/travis-worker/issues/56)
   * **Chrome browser** is pre-installed and good working on the 32bit workers (e.g. default/ruby), but on 64bit workers: https://github.com/travis-ci/travis-ci/issues/938

Open Questions: 
 * Find an elegant way to dynamically link to related Github issues (filter based on some categorization convention)
 * Is there somebody in Travis core team already preparing such kind of documentation update ?

Any comments and commits are welcome!