Added check for the output directory, if it exists clean it out before running build.
Added check for the output directory, if it exists clean it out before running build.

cloudn/cf-php-buildpack-binary-build-scripts,cloudn/cf-php-buildpack-binary-build-scripts,dmikusa-pivotal/cf-php-buildpack-binary-build-scripts,dmikusa-pivotal/cf-php-buildpack-binary-build-scripts