Use Go 1.12 (beta)

As Go 1.12 is available in App Engine standard environment in Beta (2019-03), we prefer moving from Go 1.11 to 1.12 😄

• https://cloud.google.com/appengine/docs/standard/go111/release-notes#march_25_2019
• https://cloud.google.com/appengine/docs/standard/go112/release-notes#march_25_2019

cf. #17 (closed)