Should not raise an exception when the resolver is already installed

[coreyti]

e.g.,

$ sudo meltdown install
meltdown... installing resolver config at /etc/resolver/dev
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:243:in `mkdir': File exists - /etc/resolver/dev (Errno::EEXIST)
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:243:in `fu_mkdir'
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:217:in `mkdir_p'
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:215:in `reverse_each'
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:215:in `mkdir_p'
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:201:in `each'
  from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/fileutils.rb:201:in `mkdir_p'
  from /Library/Ruby/Gems/1.8/gems/meltdown-0.1.1/bin/meltdown:42:in `install'
  from /Library/Ruby/Gems/1.8/gems/thor-0.14.6/lib/thor/task.rb:22:in `send'
  from /Library/Ruby/Gems/1.8/gems/thor-0.14.6/lib/thor/task.rb:22:in `run'
  from /Library/Ruby/Gems/1.8/gems/thor-0.14.6/lib/thor/invocation.rb:118:in `invoke_task'
  from /Library/Ruby/Gems/1.8/gems/thor-0.14.6/lib/thor.rb:263:in `dispatch'
  from /Library/Ruby/Gems/1.8/gems/thor-0.14.6/lib/thor/base.rb:389:in `start'
  from /Library/Ruby/Gems/1.8/gems/meltdown-0.1.1/bin/meltdown:86
  from /usr/bin/meltdown:19:in `load'
  from /usr/bin/meltdown:19