Drop Jenkins.getBestBuildURL() method

The ability to configure success and failure URL patterns (cf.
'success-pattern' and 'failure-pattern') obsoletes the need to
guess-by-fetching an appropriate link for the build status, which can be
extremely expensive. (Wikimedia's Zuul instance makes three HTTP requests per
invocation -- 'testReport', which 302s to 'testReport/', which 404s, and then
'consoleFull', which often runs to hundreds of kilobytes.)

Also corrects a small typo in README.rst.

Change-Id: Ib222f544c98253152a5e787ec0cdf28fa2d80cf6
Reviewed-on: https://review.openstack.org/28128
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins