Prepare release 2.12.0 (jaegertracing#269)

* Prepare release 2.12.0

Signed-off-by: Yuri Shkuro <ys@uber.com>

Preparing release 2.11.2

Signed-off-by: Yuri Shkuro <ys@uber.com>

Preparing release 2.11.1 (jaegertracing#238)

Signed-off-by: Won Jun Jang <wjang@uber.com>

Preparing release 2.11.0

Signed-off-by: Yuri Shkuro <ys@uber.com>

Prepare release 2.10.0 (jaegertracing#226)

Preparing release 2.9.0 (jaegertracing#180)

Preparing release 2.8.0

Preparing release 2.7.0 (jaegertracing#164)

Preparing release 2.6.0 (jaegertracing#132)

Preparing release 2.5.0 (jaegertracing#127)