[ci] bundle install into system gems

Git on Windows respects the MAX_PATH from the Win32 API. Since MAX_PATH
is 260 characters and the path to the Jenkin’s workspace is fairly long,
`bundle install --path=vendor/bundle` commands will fail if the
`Gemfile` contains a `git` sources.

Although `msysgit` is working on experimental long path support, there
are still various bugs in the implementation:
https://github.com/msysgit/git/pull/122