Update assertions for 'git:check' test scenario

Capistrano 'git:check' task executes a `git ls-remote -h #{repo_url}`
command to check if remote repo is accessible. If successful, command
output is a list of references and their SHAs.

Task should be successful and we're pretty sure remote repo will have a
`master` reference, so that's what we're asserting.

We're NOT asserting:
 * repo references other than master - likely to change
 * reference SHAs - will definitely change

I have reviewed and made sure the pull request complies with [CONTRIBUTING.md](https://github.com/capistrano/capistrano/blob/master/CONTRIBUTING.md).