Update Android Guide (rough and dirty)

These changes correspond to travis-ci/travis-cookbooks#337
Add note on 'test' DB no longer being present
Merge branch 'gh-pages' of github.com:travis-ci/docs-travis-ci-com into gh-pages
Link-ify the change log
Add documentation for CocoaPods caching
Add documentation for `podfile` setting
Fix cache docs for CocoaPods caching

You should cache the Pods directory, not the Podfile.
Bump to Android SDK 23.0.2

Related to travis-ci/travis-ci#2596
Improve the updates of the Android Guide

Kudos and credits to @BanzaiMan review. Thanks!
Android: recommend to specify all dependencies

This practice ensure stable build environment even if some preinstalled
components are removed from the Travis VM. Unfortunately, it will also
increase the total build time (until we find a solution to this
problem).

See discussion in travis-ci/travis-ci#2296 for full details about the
advantages and drawbacks to rely on the pre-installed components.
Merge pull request #91 from travis-ci/android-updates

Update Android Guide
Clarify that xdebug is enabled by default

This is discussed in https://github.com/travis-ci/travis-ci/issues/1697.
Merge branch 'gh-pages' of github.com:travis-ci/docs-travis-ci-com into gh-pages
Reformat instructions to disable xdebug
Rework wording

You'd add 'before_script' to '.travis.yml', really
Announce Erlang 17.1

Late July, 2014 update
Update documented Perl versions
Add snippet on why cache can become invalid

There have been reports of build failures due to VM updates
changing the language versions today on .com.

This documentation update aims to clarify this situation
for the future.

There are other improvements we can make:
1. Mention that the language updates can invalidate the existing caches
in the announcement blog post
2. Preemptively clear (some or all) caches just prior to update rollouts
(This, too, needs to be announced.)
Add commentary on 'pecl install'

I couldn't figure out how to recover from the error when
the desired version is already present on the system, so
no workaround is offered.

The exit status of 'pecl' is 1, and there is nothing noteworthy
in the commands to detect this condition and advise users.

The worst case scenario is that, when we update VM and installed
the most up-to-date package, 'pecl install' will start failing
randomly, and there is nothing we can do about it.

One may consider `pecl install X || true`, but then legitimate errors
from `pecl` will be ignored.
Merge branch 'gh-pages' into rkh-sshkey-and-encrypt-file