Add newline char at the end of annotation wrapper:

  Terminate Devtools::ANNOTATION_WRAPPER format string with a "\n"
character, so that when appending content to Gemfile and Rakefile all
lines will end the same way.


  I did the most simple and naive change here, another way might be to
implement this logic in Devtools::Site::Initializer#init_gemfile and
Devtools::Site::Initializer#init_rakefile or maybe a new method.