Drop 0.4 and 0.5?

[tkoolen]

How do you feel about dropping support for 0.4 and 0.5? Some reasons to do so:

• It would be quite annoying to get the package depwarn-free on 0.7 due to the immutable -> struct change. I've got https://github.com/tkoolen/docstringextensions.jl/tree/tk-fix-depwarns ready to be PR'd, but there's no Compat for struct because it's a parser change, and the workaround is not very desirable.
• The type system changes are pretty significant. It would be a hassle to fix Error involving SIGNATURES and UnionAll type arguments #32 while keeping support for 0.4 and 0.5.

[mortenpi]

I'm in. I should drop 0.5 support in Documenter as well. The old minor versions will continue working for people who need it on older Julia versions.