doc: update commands for fetching release PGP keys

Use https to ceph.com.  The gitweb URLs aren't as pretty, but our
cert is at least valid!

Signed-off-by: Sage Weil <sage@inktank.com>
test/crypto: fix narrowing conversion warning

warning: test/crypto.cc:49:3: narrowing conversion of ‘136’ from ‘int’ to ‘char’ inside { } is ill-formed in C++11 [-Wnarrowing]

Signed-off-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph