minor #32 Updated Composer requirements (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #32).

Discussion
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Updated Composer requirements

First of all, **we can't merge this pull request**. I've just opened to get your feedback and comments about it.

In my computer, the demo application works perfectly and I see no errors or warnings. However, if I set a PHP extension constraint in PHP, I get the following Composer errors:

### ext-pdo-sqlite

![ext-pdo-sqlite](https://cloud.githubusercontent.com/assets/73419/7061612/b4386c0c-de94-11e4-9771-e566e3b3b64f.png)

### ext-sqlite

![ext-sqlite](https://cloud.githubusercontent.com/assets/73419/7061614/b525af1c-de94-11e4-8d05-828765f3b063.png)

Commits
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a587d94 Updated Composer requirements

Author: javiereguiluz

composer.json

@@ -8,6 +8,7 @@
     },
     "require": {
         "php"                                  : ">=5.3.3",
+        "ext-pdo_sqlite"                       : "*",
         "doctrine/doctrine-bundle"             : "~1.2",
         "doctrine/doctrine-fixtures-bundle"    : "~2.2",
         "doctrine/orm"                         : "~2.2,>=2.2.3",