CAS-1347: Missing language keys prevents access should not cause a crash

I added the missing French properties.
I also replaced the French accents by their unicode values, to avoid new mistakes on encoding. I should do that for the next change on 4.0.

I also changed the EhCache version. First, I wanted to keep the 2.7.2 version, but there was some impacts on the ```cas-server-integration-ehcache``` module so I downgraded to the 2.6.6 version.