Warning user about UTF-8 console encoding

I was getting this error on Jenkins CI: 

```
+ ipa build --trace
/Library/Ruby/Gems/2.0.0/gems/shenzhen-0.5.3/lib/shenzhen/xcodebuild.rb:64:in `===': invalid byte sequence in US-ASCII (ArgumentError)
	from /Library/Ruby/Gems/2.0.0/gems/shenzhen-0.5.3/lib/shenzhen/xcodebuild.rb:64:in `settings'
	from /Library/Ruby/Gems/2.0.0/gems/shenzhen-0.5.3/lib/shenzhen/commands/build.rb:51:in `block (2 levels) in <top (required)>'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/command.rb:155:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/runner.rb:402:in `run_active_command'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/runner.rb:78:in `run!'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/delegates.rb:7:in `run!'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.1.5/lib/commander/import.rb:10:in `block in <top (required)>'
Build step 'Execute shell' marked build as failure
```

I solved that adding ```export LC_ALL="en_US.UTF-8"``` to my script, so I'm summiting this pull request to warn users about this error, CocoaPods are doing a similar approach.