Run the test suite using GitHub Actions

Using GitHub Actions has a few advantages over using Travis CI:

* More jobs can be run in parallel
* All CONSUL repositories on GitHub will be configured automatically

Besides, Travis have recently changed their policy twice. First, they
announced their site for free software projects would be shut down but
free software projects could still use their site for private projects.
And then, they limited the usage of their services for free software
projects.

Just like we used to do with Travis, we're enabling builds for pull
requests but not for pushed branches.

We're also building the master branch. Even if we never push to the
master branch directly, we're aware other CONSUL repositories do, so
we're running the tests for this case.

.github/ISSUE_TEMPLATE/flakies.md

@@ -3,7 +3,7 @@ Tests that fail randomly are called "flakies", this one seems to be one:
 
 **Randomized seed:** FILL_WITH_RANDOM_SEED
 
-**Travis failed build:** FILL_WITH_TRAVIS_FAILED_BUILD_LINK
+**Failed action:** FILL_WITH_A_LINK_WHERE_THE_FAILURE_IS_SHOWN
 
 **Failure:**
 
@@ -26,7 +26,7 @@ do
 done
 ```
 
-You can also try running a single spec in Travis:
+You can also try running a single spec:
 Add option `:focus` to the spec and push your branch to Github, for example:
 ```ruby
 scenario 'Show', :focus do