Fix warning: circular require considered harmful
Fix warning: circular require considered harmful

Running the riemann-tools test suite emit this warning:
lib/riemann.rb:15: warning: lib/riemann.rb:15: warning: loading in progress, circular require considered harmful - lib/riemann/client.rb

The README.md says one should require 'riemann/client', so let
'riemann/client' load 'riemann', and stop loading 'riemann/client' from
'riemann'.

Fix the warning.

riemann/riemann-ruby-client