Update composer.json

Bump dependency of doctrine common for symfony 2.1 users.
merged branch guilhermeblanco/patch-3 (PR #202)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #202).

Commits
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3fa2c5c Update composer.json

Discussion
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Update composer.json

Bump dependency of doctrine common for symfony 2.1 users.