Don't accept Formula objects as dep specs

It was pointed out that this isn't used at all.
libgit2 0.18

Closes #19261.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
MongoDB 2.4.2

Closes #19266.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
mongodb: use arch requirement
libev 4.15

Closes #19273.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>