bugfix #176 Explicitly require "doctrine/common" (phansys)

This PR was merged into the 3.x branch.

Discussion
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|Q            |A  |
|---          |---|
|Branch       |3.x|
|Bug fix?     |yes|
|New feature? |no |
|BC breaks?   |no |
|Deprecations?|no |
|Tests pass?  |yes|
|Fixed tickets|n/a|
|License      |MIT|
|Doc PR       |   |

Commits
-------

77b08fd Explicitly require "doctrine/common"

composer.json

@@ -26,6 +26,7 @@
     ],
     "require": {
         "php": "^5.6 || ^7.0",
+        "doctrine/common": "^2.7",
         "doctrine/doctrine-bundle": "^1.4 || ^2.0",
         "doctrine/orm": "^2.4.8",
         "knplabs/knp-paginator-bundle": "^2.6 || ^3.4 || ^4.0 || ^5.0",