Remove coveralls.io integration

[jarifibrahim]

Coverall.io integration fails frequently which leads to a failed build. We should remove it.

See example build https://travis-ci.org/dgraph-io/badger/jobs/636936733?utm_medium=notification&utm_source=github_status

badger/.travis.yml

Lines 24 to 29 in 5870b7b

	before_script:
	- go get github.com/mattn/goveralls
	script:
	- bash contrib/cover.sh $HOME/build coverage.out || travis_terminate 1
	- goveralls -service=travis-ci -coverprofile=coverage.out || true
	- goveralls -coverprofile=coverage.out -service=travis-ci

[janisz]

How about replacing it with https://codecov.io/
Integration is much simpler

after_success:
- bash <(curl -s https://codecov.io/bash)

[vardhanapoorv]

@janisz the code coverage isn't a reliable metric, hence it would be better to remove it.
Feel free to send a PR.

[janisz]

@vardhanapoorv test are not working :(

[jarifibrahim]

Hey @janisz . I am aware of the test failure. There seems to be some configuration issue with the build. We'll fix it soon :)

Thank you for your PR.