Fix misleading advice to add 'memcache' to Gemfile

[#5539 state:committed]

Commit 57144388 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: NSantiago Pastorino <santiago@wyeworks.com>

activesupport/lib/active_support/cache/mem_cache_store.rb

 begin
   require 'memcache'
 rescue LoadError => e
-  $stderr.puts "You don't have memcache installed in your application. Please add it to your Gemfile and run bundle install"
+  $stderr.puts "You don't have memcache-client installed in your application. Please add it to your Gemfile and run bundle install"
   raise e
 end
 require 'digest/md5'