replaced erlang's random library with the new rand

[ifoo]

Compiling on Erlang/OTP 19 yield the following deprecation warnings:

===> Compiling rabbit_common
./deps/rabbit_common/src/gen_server2.erl:770: Warning: random:uniform_s/2: the 'random' module is deprecated; use the 'rand' module instead
src/gen_server2.erl:770: Warning: random:uniform_s/2: the 'random' module is deprecated; use the 'rand' module instead

./deps/rabbit_common/src/rabbit_misc.erl:1169: Warning: random:seed/3: the 'random' module is deprecated; use the 'rand' module instead
./deps/rabbit_common/src/rabbit_misc.erl:1174: Warning: random:uniform/1: the 'random' module is deprecated; use the 'rand' module instead
src/rabbit_misc.erl:1169: Warning: random:seed/3: the 'random' module is deprecated; use the 'rand' module instead
src/rabbit_misc.erl:1174: Warning: random:uniform/1: the 'random' module is deprecated; use the 'rand' module instead

The pull request replaces the deprecated random module with the new rand module.