Fix composite primary key introspection on Sqlite

Running the Sqlite testsuite I get the following error:

```bash
There was 1 failure:

1) Doctrine\Tests\DBAL\Functional\Schema\SqliteSchemaManagerTest::testListTableIndexes
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
 Array (
     0 => 'id'
-    1 => 'other_id'
 )

/home/deeky/dev/doctrine/dbal/tests/Doctrine/Tests/DBAL/Functional/Schema/SchemaManagerFunctionalTestCase.php:272
```

This is due to the schema manager not correctly building composite primary keys on Sqlite. I don't know why this does not fail on Travis but accoding to the [official documentation](http://www.sqlite.org/pragma.html#pragma_table_info) this approach is the correct implementation.
I did not add a dedicated test for this as there obviously already is one that covers this.