Revert "Default nginx version changed to 1.15.x."
Revert "Default nginx version changed to 1.15.x."

This reverts commit f6f5bf4d9b4e119110fc3ecf16692bf087213c00.

Compile extensions seem unable to handle a wildcard default version.

cf-identity/php-buildpack,cloudfoundry/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack,cloudfoundry/php-buildpack,cf-identity/php-buildpack