Add `s` dependency in the main esup.el

[benley]

I guess package.el only pays attention to this.

Fixes #81

[codecov-commenter]

Codecov Report

Merging #82 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   37.87%   37.87%           
=======================================
  Files           2        2           
  Lines         396      396           
=======================================
  Hits          150      150           
  Misses        246      246           

[sergeyklay]

Thank you