minor #1618 Updated PHP compatibility information in README (derrabus)

This PR was merged into the 2.3.x-dev branch.

Discussion
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Updated PHP compatibility information in README

This PR documents the new version constraints from #1607.

Commits
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8f504581 Updated PHP compatibility information in README.

README.rst

@@ -23,7 +23,7 @@ components`_:
 
     $app->run();
 
-Silex works with PHP 5.5.9 or later.
+Silex works with PHP 7.1.3 or later.
 
 Installation
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