Log Message:
Change RUBY_VER_DIR from RUBY_VERSION to RUBY_API_VERSION on Ruby 2.1 and
2.2.  It affects ruby 2.1 only.

* Binary packages' file name start with "ruby21-" instead of \ 
"ruby215-".
* ${PREFIX}/include/ruby-2.1.5 are changed from ${PREFIX}/include/ruby-2.1.5
  to ${PREFIX}/include/ruby-2.1.0.
* ${PREFIX}/lib/ruby/2.1.5 and ${PREFIX}/lib/ruby/gem/2.1.5 are changed
  to ${PREFIX}/lib/ruby/2.1.0 and ${PREFIX}/lib/ruby/gem/2.1.0.