Load Rakefile only once.

Currently Rakefile is loaded more than one time when multiple rake guards are specified in the Guardfile. For example:
````ruby
# Guardfile

guard "rake", :task => :foo do
  watch("foo")
end

guard "rake", :task => :bar do
  watch("bar")
end
````

Will run ````:foo```` and ````:bar```` tasks twice each time. This pull request fixes that.