Allow global yarn install of pr-bumper

[sandersky]

pr-bumper works when using using npm install -g pr-bumper but not when using yarn global add pr-bumper. I suspect the yarn global bin isn't on the path while the npm global bin is, but it'd be nice if pr-bumper just supports both ways out-of-box.

[job13er]

pr-bumper doesn't have any control or care about how it's installed or called. If the consumer wants to install it from git or with npm or with yarn, they have to make sure that the path to where they installed it is in their PATH if they want to invoke it without including exactly where it was installed. I'm not sure what pr-bumper could or should do as a project to ensure the environment in which it's installed is set up correctly, as it has no visibility or control over the PATH environment in which it's invoked.