Fix warnings on Ruby 2.1.2

This PR suppresses following warnings.
```sh
% bundle exec rake
Loaded suite /home/yuya/.rvm/gems/ruby-2.1.2/bin/rake
Started
/home/yuya/github/ruby-git/lib/git/path.rb:10: warning: File.exists? is a deprecated name, use File.exist? instead
/home/yuya/github/ruby-git/lib/git/path.rb:10: warning: File.exists? is a deprecated name, use File.exist? instead
/home/yuya/github/ruby-git/tests/units/test_archive.rb:18: warning: File.exists? is a deprecated name, use File.exist? instead
...
```

We can replace `File.exists?` to `File.exist?` since Ruby 1.8.7.
```sh
% ruby -ve 'p File.exist?("/tmp")'
ruby 1.8.7 (2013-06-27 patchlevel 374) [x86_64-linux]
true
```

In 33f100ffe8d009314c4f209778cfb95b98b7c57c , ruby-git already uses `File.exist?` 1 line.
But 29 lines uses `File.exists?` yet.
This PR fixes it.
```sh
% git grep -n File.exist?
lib/git/lib.rb:484:      if File.exist?(filename)
% git grep -n File.exists? | wc -l
29
% git grep -n File.exists?
lib/git/base.rb:449:      if File.exists?(file)
lib/git/base.rb:455:      self.lib.apply_mail(file) if File.exists?(file)
...
```