10.1.0 breaks compatibility with git < 1.7.3

Summary

gitlab-runner 10.1.0 started to use the --local option of git-config (commit d60067d7, which was later changed with ac5ab2fc), which breaks compatibility with RHEL/CentOS 6, because the --local option was introduced with git 1.7.3 (https://github.com/git/git/commit/57210a6), and RHEL/CentOS 6 are shipped with git 1.7.1.

Steps to reproduce

Install gitlab-runner 10.1.0 or later on CentOS 6.

Actual behavior

Running with gitlab-runner 10.1.0 (c1ecf97f)
  on xxx (xxx) (xxx)
Using Shell executor...
Running on xxx...
Fetching changes...
error: unknown option `local'
usage: git config [options]

Expected behavior

Work with git version < 1.7.3