Update java example
PHP buildpack guide
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Rewrite Add-on Credentials

Removed references to file name "creds.json"
Added section about SET_ENV_VARS
Rewrite Python Helloworld
Use new example apps
Add 'Helloworld in Silex' to PHP guides
Update ruby guides
Add notes for Django and Django migration guide
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Update Guides/PHP/Buildpack.md
Update PHP guides