Update Silex to the 1.1.x version range
Update the project for Propel 2

Update the project to be up to date with the last Propel release. This
commit includes the following changes:

  - Fix folder structure for this version : Propel 1.x used to generate
    a build folder with the classes and conf subdirectories. These
    folders are now generated-classes and generated-conf
  - We now check whether the Propel class is defined under its Runtime
    namespace
  - The path to load Propel has also been updated
  - Update the travis file to run on PHP 5.4 and 5.5 but not 5.3
  - Also now, the package requires at least PHP 5.4

Also move the dummy files to .gitkeep files.
Allow users to use Silex all 1.x versions
Add installation instructions to the README file