Issue title: Wrong exit code in install.sh 
 Issue body: New installations of YouCompleteMe will fail, if the exit code of install.sh is checked.

See: https://github.com/Valloric/YouCompleteMe/commit/6572635c67df24d8f4955d10ffe9bdede2f84bc1

 Comments: 
 Comment 0: Fixed by 6b7e7361571bf6b54dc78c46fe76849c64bb59d5

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