Add a basic functional test for mkcert, for #136

[rfay]

#136 points out that mkcert is completely lacking tests.

This PR does a very basic functional test building, testing, and using the mkcert binary

• Build mkcert for current OS
• mkcert -install
• mkcert localhost to get a cert/key
• Run a server
• Hit the server and look for errors

The go test -v is added to the .travis.yml and passes on the single current linux environment (Ubuntu 16.04 with sudo enabled).

A next step would be a matrix including Travis's other available Ubuntu versions, macOS, and WIndows if we can get past the confirmation step in mkcert -install.

[rfay]

@FiloSottile what a day you had today, congratulations and thanks.

I wanted to mention that for my job's "upstream contribution goal" I said that I'd try to take on pushing forward tests for mkcert. If you have specific direction you'd like to give, that's great. If you want to start with this one and I'll propose others, that's great too. But this is definitely on my radar.

Updated to current v1.4.1

[rfay]

@FiloSottile I'd love your direction on tests, and would be pleased to contribute with a set of tests. I do think it needs your blessing and a tiny bit of direction to put more work into. Please let me know if you'd like to proceed or if you have other ideas for the direction of automated testing. I"m happy to contribute with an alternate direction as well.