Drop support for 0.4 and 0.5, fix type and immutable depwarns

[tkoolen]

Awaiting #35 decision.

BTW, I also tried to fix the current_module depwarns using @__MODULE__, but that didn't work straight away; not sure why yet.

Closes #35

[mortenpi]

LGTM, many thanks! I assume it's ready to go?

[mortenpi]

As for current_module() -- I was looking into this a while back. The issue is that current_module() and @__MODULE__ are not equivalent, since current_module() returns the module where the function is called, but @__MODULE__ gives the module where the function is defiend. Here's a simple example of the difference:

module Foo
foo() = current_module(), @__MODULE__
end
Foo.foo()

If I remember correctly, template_hook() causes issues, since it has to figure out in which module it is being called. I didn't figure out how the get that information in 0.7 though. It needs to be fixed, but it doesn't have to happen in this PR.

[tkoolen]

Yep, ready to go as far as I'm concerned. Thanks!

[mortenpi]

Nightly test failures are unrelated, due to a bug on Julia master as far as I can tell.