Improved mirror support

- Don't overwrite install URLs for yaml, openssl or rubygems with `RUBY_BUILD_MIRROR_URL`
- Use anchor for mirror checksum to match non-mirror URIs

example:

    $ RUBY_BUILD_MIRROR_URL="http://www.mirrorservice.org/sites/ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p448.tar.gz" \
      rbenv install 1.9.3-p448

    Downloading yaml-0.1.4.tar.gz...
    -> http://pyyaml.org/download/libyaml/yaml-0.1.4.tar.gz
    Installing yaml-0.1.4...
    Installed yaml-0.1.4 to /Users/matt/.rbenv/versions/1.9.3-p448

    Downloading ruby-1.9.3-p448.tar.gz...
    -> http://www.mirrorservice.org/sites/ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p448.tar.gz#a893cff26bcf351b8975ebf2a63b1023
    Installing ruby-1.9.3-p448...
    Installed ruby-1.9.3-p448 to /Users/matt/.rbenv/versions/1.9.3-p448