Added badges for Is it maintained, codecov and coveralls

[nisargthakkar]

Adding the badges to address #705 and #706

The badges added are:
Is it maintained: Resolution time
Is it maintained: Percentage of open issues
Codecov: Code coverage
Coveralls: Code coverage

[nisargthakkar]

@carols10cents Can you review this PR?

Corresponding PR in the cargo repository: rust-lang/cargo#4038

[nisargthakkar]

Hey @carols10cents Can you retrigger the travis ci build? These errors no longer pop up locally when I do cargo test. Plus the line numbers that travis suggests has errors doesn't contain the relevant code

[nisargthakkar]

I don't think Travis CI is taking the latest push for building as the error reported is:

error: no associated item named update_crate_old found for type cargo_registry::Badge in the current scope
--> src/tests/badge.rs:222:5
|
222 | Badge::update_crate_old(req.tx().unwrap(), &krate, badges).unwrap();
| ^^^^^^^^^^^^^^^^^^^^^^^

But this statement has now moved to L216. Similarly, all other errors are also off by similar line counts

@carols10cents What can I do to run the build with the latest code changes?

[carols10cents]

I think the problem is that I pushed code that removed the Badge::update_crate_old function :-X I'm going to try fixing up your branch (since I broke it) and pushing a new commit.

[nisargthakkar]

Thanks @carols10cents

[alexcrichton]

ping @carols10cents, mind taking a look at this?

[carols10cents]

Whoops, crates.io has been on my back burner lately. Will hopefully review this tonight!

[carols10cents]

Looks great, works great! Tried out all the badges locally and everything worked as I would expect 👍 👍 👍 Thank you!!!!