Add cover directory from 'mix test --cover' to default .gitignore
Add cover directory from 'mix test --cover' to default .gitignore

ggcampinho/elixir,beedub/elixir,antipax/elixir,antipax/elixir,gfvcastro/elixir,ggcampinho/elixir,michalmuskala/elixir,pedrosnk/elixir,pedrosnk/elixir,joshprice/elixir,kelvinst/elixir,lexmag/elixir,gfvcastro/elixir,beedub/elixir,elixir-lang/elixir,kimshrier/elixir,kimshrier/elixir,kelvinst/elixir,lexmag/elixir