Only run tmux specs if tmux is installed

spec/foreman/tmux_engine_spec.rb

@@ -35,6 +35,7 @@ describe "Foreman::TmuxEngine", :fakefs do
     end
   end
 
+  if system("which tmux")
     describe "start" do
       before do
         write_procfile
@@ -71,3 +72,4 @@ describe "Foreman::TmuxEngine", :fakefs do
       end
     end
   end
+end