Add code coverage for mac/linux Travis builds

[StanleyGoldman]

While developing #1866

I attempted to setup code coverage for travis but I had issues determining the path of coverlet when installed with dotnet install coverlet. I decided to reduce my scope and not include Travis CI.

Other notes

Commit where I attempted it

octokit.net/build/Tasks/CodeCoverage.cs

Lines 24 to 29 in 8904a34

	context.Coverlet(project, new CoverletToolSettings()
	{
	Configuration = context.Configuration,
	Framework = "netcoreapp2.0",
	Output = output
	});

Builds

https://ci.appveyor.com/project/github-windows/octokit-net/build/0.33.0-PullRequest.1866+10.build.2870

https://travis-ci.org/octokit/octokit.net/jobs/426700235

========================================
CodeCoverage
========================================
Executing task: CodeCoverage
Executing Code Coverage Project Octokit.Tests...
An error occurred when executing task 'CodeCoverage'.
Error: Coverlet: Could not locate executable.
The command "./build.sh --linksources=true --verbosity=verbose" exited with 1.

https://travis-ci.org/octokit/octokit.net/jobs/426700236

[shiftkey]

Closing this out as unimportant given how .NET Core is cross-platform and the differences between the OSes is so minor these days. As long as something is submitting code coverage stats I'm good.