ESL-89: fix install path and single_command require

libs/esl/ruby/Makefile

 RUBY:=ruby
 LOCAL_LDFLAGS=$(shell $(RUBY) -rrbconfig -e 'puts RbConfig::CONFIG["LIBRUBYARG"]')
 LOCAL_CFLAGS=$(shell $(RUBY) cflags.rb)
-RUBY_GEM_DIR=$(shell $(RUBY) -e 'puts Gem.dir')
+RUBY_GEM_DIR=$(shell $(RUBY) -e 'puts RbConfig::CONFIG["rubylibdir"]')
 
 all: ESL.so
 

libs/esl/ruby/single_command.rb

 #! /usr/bin/ruby
-require_relative './ESL' 
+require 'ESL' 
 
 HOST     = '127.0.0.1'.to_s 
 PORT     = '8021'.to_s