Update path creation/handling for artifacts

[mononym]

When attempting to run mix coveralls -o public/cover to generate coverage docs for usage in GitLab pages I get the following error: ** (File.Error) could not make directory "./public/cover": no such file or directory

By simply adding the -p options to mkdir this ceases to be a problem. Hence I switched both File.mkdir!/1 calls to File.mkdir_p!/1

[coveralls]

Coverage increased (+0.03%) to 89.63% when pulling df89d71 on mononym:master into e14b766 on parroty:master.

[parroty]

Thanks!