Use go 1.9

This implements the following changes, related to #823 (closed) and https://gitlab.com/gitlab-com/infrastructure/issues/3412

• Build gitaly using Go 1.8 and Go 1.9
• Add Golang 1.9 tests to the matrix, for Git 2.8 and 2.14
• Archive go binary artifacts for Go 1.8 and Go 1.9, for six months, for tagged builds