Removed isUser()
minor #1257 Issue #1192 - removed isUser() (jaspernbrouwer)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Issue #1192 - removed isUser()

This PR solves issue #1192, it removes `isUser()` from `FOS\UserBundle\Model\UserInterface` and implementing classes.

Commits
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10f2b23 Removed isUser()
Updated the changelog