Android: update 'platform-tools' before 'tools'

This is a bugfix for travis-ci/travis-cookbooks#403.
Otherwise, the latest SDK tools cannot be installed:

```
Installing Archives:
  Preparing to install archives
  Skipping 'Android SDK Tools, revision 23.0.5'; it depends on 'Android
  SDK Platform-tools, revision 21' which was not installed.
  Done. Nothing was installed.
```

References and Credits:
 - https://github.com/travis-ci/travis-ci/issues/2931
 - Special thanks to @ardock for stressing this problem out
(https://github.com/travis-ci/travis-ci/issues/2848#issuecomment-64977198)