Update core.rb with "# encoding: utf-8"

ArgumentError: invalid byte sequence in US-ASCII
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/run_loop-1.0.9/lib/run_loop/core.rb:120:in `gsub'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/run_loop-1.0.9/lib/run_loop/core.rb:120:in `run_with_options'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/run_loop-1.0.9/lib/run_loop/core.rb:679:in `run'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/calabash-cucumber-0.11.3/lib/calabash-cucumber/launcher.rb:724:in `block in new_run_loop'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/calabash-cucumber-0.11.3/lib/calabash-cucumber/launcher.rb:722:in `times'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/calabash-cucumber-0.11.3/lib/calabash-cucumber/launcher.rb:722:in `new_run_loop'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/calabash-cucumber-0.11.3/lib/calabash-cucumber/launcher.rb:597:in `relaunch'
	from /Users/eduardo/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/calabash-cucumber-0.11.3/lib/calabash-cucumber/core.rb:838:in `start_test_server_in_background'
	from (irb):1
	from /Users/eduardo/.rbenv/versions/2.1.1/bin/irb:11:in `<main>'