Well, this is a common problem with MySQL packages in CentOS 6. Yum tries to satisfy the requirements by setting mysql-libs-5.1.69-1.el6_4.x86_64 . This package conflicts with files from the MySQL55-server-5.5.32-1.cp1136.x86_64 .
It is possible to fix this without breaking the rpm database or removing mysql55, just download mysql compatible libraries from mysql for el6 (CentOS and RHEL6 are fully binary compatible). Get MySQL-shared-compat-5.5.32-2.el6.x86_64.rpm (just Google for it) and install this rpm using: rpm -ihv MySQL-shared-compat-5.5.32-2.el6.x86_64.rpm
After that, you can install ReviewBoard without yum trying to install mysql51 libs to satisfy the dependencies.
Here is the link to the official package provided by MySQL: collaboration with MySQL
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