Revert "Use :pgroup option when starting subprocesses"

This reverts commit 308fb9bc94.

plug.vim

@@ -1004,22 +1004,6 @@ function! s:update_ruby()
     %["#{arg.gsub('"', '\"')}"]
   end
 
-  require 'rubygems'
-  require 'thread'
-  require 'fileutils'
-  require 'timeout'
-
-  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('1.9')
-    def popen cmd
-      IO.popen(cmd, :pgroup => true)
-    end
-    def killall pid
-      Process.kill 'TERM', - pid rescue nil
-    end
-  else
-    def popen cmd
-      IO.popen(cmd)
-    end
   def killall pid
     pids = [pid]
     unless `which pgrep 2> /dev/null`.empty?
@@ -1033,7 +1017,10 @@ function! s:update_ruby()
     end
     pids.each { |pid| Process.kill 'TERM', pid.to_i rescue nil }
   end
-  end
+
+  require 'thread'
+  require 'fileutils'
+  require 'timeout'
   running = true
   iswin = VIM::evaluate('s:is_win').to_i == 1
   pull  = VIM::evaluate('s:update.pull').to_i == 1
@@ -1100,7 +1087,7 @@ function! s:update_ruby()
           File.unlink tmp rescue nil
         end
       else
-        fd = popen(cmd).extend(PlugStream)
+        fd = IO.popen(cmd).extend(PlugStream)
         first_line = true
         log_prob = 1.0 / nthr
         while line = Timeout::timeout(timeout) { fd.get_line }