merge revision(s) 49015,49016: [Backport #10647]

	* tool/rbinstall.rb: skip installing bundle gems if zlib is unavailable.
	  [Bug #10647] [ruby-dev:48787]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@49031 b2dd03c8-39d4-4d8f-98ff-823fe69b080e