Link to 5.1 news entry in changelog

PHP-DI#315 Changelog for v5.1

Fix Container::call() to work with embedded definitions in parameters

The latest version of the Invoker library broke a buggy behavior in PHP-DI. This commit fixes the root issue in PHP-DI.

Merge pull request PHP-DI#290 from PHP-DI/performance-improvements

Performance improvements

Fixes PHP-DI#270 Add a "Twitter" button in the footer

Fixes PHP-DI#270 Add a "Twitter" button in the footer

Tiny formatting update

Merge pull request PHP-DI#284 from pulyaevskiy/improve-error-message-…

…on-undefined-parameter

Improve exception message on unknown method argument

Allow failures for HHVM

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