[#5539 state:committed]
Commit 57144388f removed the hard-coded dependency on the memcache-client
gem, and added this warning advising people to install it if needed. The
problem is, however, that if people follow the advice literally and install
the 'memcache' gem, they will wind up with a completely different thing,
which is not API compatible with the memcache-client gem and which Rails
can't work with.
So, be explicit and tell users to install the 'memcache-client' gem.
Signed-off-by: Santiago Pastorino <santiago@wyeworks.com>