ruby-lex.rb: fix for label
* lib/irb/ruby-lex.rb (identify_identifier): treat identifier just
followed by a colon as a lable. this is not a precise solution
but enough for the time being. [ruby-core:78526] [Bug #13012]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@58086 b2dd03c8-39d4-4d8f-98ff-823fe69b080e