It is possible that one of the other assemblies is referencing sitecore 6 dll. You could override it in your configuration to use sitecore 7 like this.
<configuration>
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="Sitecore.Kernel" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="5.0.0.0-6.6.0.0" newVersion="7.1.0.0" />
</dependentAssembly>
<dependentAssembly>
</runtime>
hope this helps.