common.mk: relax dependency

* common.mk (lib/unicode_normalize/tables.rb): should depend on
  Unicode data files only when downloading.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48385 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
tables.rb: add

* lib/unicode_normalize/tables.rb: commit not to download and
  convert Unicode data files every time.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48386 b2dd03c8-39d4-4d8f-98ff-823fe69b080e