Add placeholders to reserve ebin dirs in crypto and os_mon appdirs
Don't check ftp.gnu.org's https cert when fetching nettle

It appears that Ubuntu doesn't ship with the correct cert(s).
Add nettle-dev to packages, and remove erroneous build-dep
Remove .gitignore files in ebin dirs; this breaks railing

Revert "Add placeholders to reserve ebin dirs in crypto and os_mon appdirs"

This reverts commit fdf3f696e414e56195bd753a5a240d990846d027.
create ebin subdir in crypto and os_mon appdirs