Some fixes in 2.1.0 preview post

Remove duplicate header; change 'Rubygem' to 'RubyGems'; small fixes.
Merge pull request #308 from ruby/fix-post

Some fixes in 2.1.0 preview post
Some fixes in 2.1.0 preview post (es)
Translate 2.1.0 preview post (de)
Translate Ruby 2.1.0preview1 (vi)