detect if Composer failed to include any required packages

This script displayed success message, even if Composer comand failed (e.g. "Installation failed, reverting ./composer.json to its original content. 
PFXPHP-CLI files have been successfully installed. 
To test PFXPHP-CLI ...")
This update prints success/failure message depending on Composer output.
detect if Composer returned unsuccessful exit code

update according to [scribu's suggestion](https://github.com/wp-cli/wp-cli.github.com/pull/23#issuecomment-24018000)
detect if Composer returned unsuccessful exit code

missed removing extra echo in last commit
Merge pull request #23 from smhmic/patch-1

detect if Composer failed to include any required packages