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
Use a more idiomatic `apt-get install` method
Replace an assertion without implementation with the one that works
Delete duplicate cucumber assertion that is unused
Update 'checking linked files' cucumber scenarios

'Checking linked files' scenarios were passing, but the steps were not
defined at all.
This commit defines the required steps.
Update 'symlink linked files' cucumber scenario

When `deploy:symlink:linked_files` task is executed, only the
release path (containing the timestamp) is created.
After the task is done, it is hard to navigate over that release path
(again, because it's a timestamp) to perform the assertions.

To solve the issue, we're additionally creating a current path because
the assertion is much more easily done via `current_path`.
Fix README, fixes #1055