Don't define MirrorCommand if already defined.

Hi.  I found that "gem mirror" subcommand doesn't work.

```
% bin/ruby -v
ruby 2.2.0dev (2014-10-10 trunk 47863) [x86_64-linux]
% bin/gem mirror
ERROR:  Install the rubygems-mirror gem for the mirror command
```

gem mirror subcommand needs rubygems-mirror gem.
So I installed it but the error still occur.


```
% bin/gem install rubygems-mirror
Fetching: net-http-persistent-2.9.4.gem (100%)
Successfully installed net-http-persistent-2.9.4
Fetching: rubygems-mirror-1.0.1.gem (100%)
Successfully installed rubygems-mirror-1.0.1
Parsing documentation for net-http-persistent-2.9.4
Installing ri documentation for net-http-persistent-2.9.4
Parsing documentation for rubygems-mirror-1.0.1
Installing ri documentation for rubygems-mirror-1.0.1
Done installing documentation for net-http-persistent, rubygems-mirror after 0 seconds
2 gems installed
% bin/gem mirror
ERROR:  Install the rubygems-mirror gem for the mirror command
```

Gem::Commands::MirrorCommand#execute shows the error message.
It seems that this method is defined by rubygems-mirror gem but
redefined by rubygems/commands/mirror_command.rb.

```
% bin/ruby -v bin/gem mirror
ruby 2.2.0dev (2014-10-10 trunk 47863) [x86_64-linux]
/home/ruby/tst1/lib/ruby/2.2.0/rubygems/commands/mirror_command.rb:4: warning: method redefined; discarding old initialize
/home/ruby/tst1/lib/ruby/gems/2.2.0/gems/rubygems-mirror-1.0.1/lib/rubygems/mirror/command.rb:8: warning: previous definition of initialize was here
/home/ruby/tst1/lib/ruby/2.2.0/rubygems/commands/mirror_command.rb:13: warning: method redefined; discarding old description
/home/ruby/tst1/lib/ruby/gems/2.2.0/gems/rubygems-mirror-1.0.1/lib/rubygems/mirror/command.rb:12: warning: previous definition of description was here
/home/ruby/tst1/lib/ruby/2.2.0/rubygems/commands/mirror_command.rb:19: warning: method redefined; discarding old execute
/home/ruby/tst1/lib/ruby/gems/2.2.0/gems/rubygems-mirror-1.0.1/lib/rubygems/mirror/command.rb:26: warning: previous definition of execute was here
...
ERROR:  Install the rubygems-mirror gem for the mirror command
```

So it can be fixed by rubygems/commands/mirror_command.rb doesn't redefine the class if it is already defined.

After the fix applied, the error gone.

```
% bin/gem mirror
ERROR:  While executing gem ... (RuntimeError)
    Config file /home/akr/.gem/.mirrorrc not found
```