Consider switching from coveralls.io to codecov.io

[jodh-intel]

I've just come across codecov.io. It's a code coverage service that is free for OSS projects and supports golang (https://docs.codecov.io/docs/supported-languages).

Here's an example of the sort of feedback you get on github with this service enabled:

• Fix SEGV when do bwipe in callback vim/vim#2107 (comment)

And here's an example of the rather bling project summary page you get:

• https://codecov.io/gh/vim/vim

Related: #23, #9.