Make legislation proposals random seed more robust

Using a number with only two decimals means the seed is going to be the
same 1% of the time. Using ruby's default value for random numbers makes
it almost impossible to generate the same seed twice.

Using `rand` also simplifies the code, and it's what we use in the
budget investments controller.

app/controllers/legislation/processes_controller.rb

@@ -117,7 +117,7 @@ class Legislation::ProcessesController < Legislation::BaseController
     end
 
     def set_random_seed
-      seed = (params[:random_seed] || session[:random_seed] || (rand(99) / 100.0)).to_f
+      seed = (params[:random_seed] || session[:random_seed] || rand).to_f
       seed = (-1..1).cover?(seed) ? seed : 1
 
       session[:random_seed] = seed