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
@@ -70,4 +71,5 @@ describe "Foreman::TmuxEngine", :fakefs do
         output.should =~ /bravo\.1.+\.\/bravo: No such file or directory/
       end
     end
+  end
 end