Update dev guide to reflect that we can use the "Big Green Button" on github

The section in the developer guide about Merging still discourages devs from using the "big green button" on github. That's out of date now: it's probably better to use it, because you'll be sure that you're rebased against master and travis completed. I don't know that we have any repositories for which it's not safe to use: if so, we could either document them, or list the conditions under which we shouldn't use it.

Please update that paragraph to reflect this.