add linux github workflow

[sergiud]

Since AppVeyor and Travis Builds ceased to work, this PR adds a corresponding Github workflow for testing glog under Ubuntu. This is a continuation of #720 with some changes reworked.

More specifically:

• Use latest Google Test and Google Mock
• Update the badges
• Generate coverage

@drigz What options are there for coverage analysis? Can codecov or coveralls be enabled for the repo?

[drigz]

Sorry, I'm afraid I've not used GitHub actions or any of those coverage tools before, my knowledge is all from the days of running make coverage and inspecting a text file...

[sergiud]

@drigz Is there someone who can enable Codecov for glog? Codecov is being used by some Google repos and coveralls is used by even more Google repos.

[drigz]

I couldn't find an explanation on how to do it, but I signed into coveralls.io and "enabled" this repo. I also found this doc about the GitHub Action: https://github.com/marketplace/actions/coveralls-github-action - is that sufficient for you to enable it here?

[sergiud]

@drigz Thanks! Enabling the coveralls github action should be sufficient.

[sergiud]

@drigz Seems to be working just fine now. Do we want notifications in PRs (e.g., as seen here)? In that case, read-only access to Coveralls bot should be given.

[drigz]

I "invited" the bot, let's see if the comments appear.

[sergiud]

Possibly a new build must be triggered as well. Let me try that.

[sergiud]

Looks like comments are only added if a baseline coverage exists which is currently not the case.