Update README links to production treeherder/issues

There is now a production treeherder instance, so link to that rather than dev. The treeherder-service Github issues tracker has been superseded by the new Bugzilla component, with issues due to be switched off soon (https://bugzilla.mozilla.org/show_bug.cgi?id=1032820).
Merge pull request #2 from edmorley/patch-1

Update README links to production treeherder/issues