Update required Go version

There was an update to 1.10 that changed coverage tests,
so you can use `-coverprofile` with multiple packages.

golang/go#6909

I had 1.9.x on my system so cover didn't work but passed in CI,
where we were using 1.10.x -- so update that assumption here.

README.md

@@ -4,7 +4,7 @@ This project is the seed for CircleCI's new command-line application.
 
 ## Requirements
 
-* Go 1.9+
+* Go 1.10+
 * Make
 * ...