Wrong error message when composer.phar do not exists on custom config path.

[yvoyer]

This issue was caused, when the following steps are encountered:

• When the composer.phar do not exists at the root
• composer.json file defines a different path to the bin

composer.json ie.

    "require-dev": {
        "humbug/humbug": "~1.0@dev"
    },
    "config": {
        "bin-dir": "bin/"
    }

When this happens, and you run humbug, we get the following error:

[Humbug\Exception\RuntimeException]                                                    
  Unable to locate a PHPUnit executable on local system. Ensure that PHPUnit is installed and available.

Putting the composer finder outside the try catch, now show the right errors about the composer missing.

[yvoyer]

@padraic, I'm not sure why travis builds do not pass, while on my machine it does. Do you have a composer installed globally?

[padraic]

Need to be rechecked to current master and/or with composer installed in various ways (per issue description) to see if reported issue can be reproduced.

With conflicts resolved, the current PR produces composer not found RuntimeExceptions for 7 behat scenarios.