Update PHP 7.1.12

[kubosho]

Failed to install system packages when deployed this app.

-----> PHP app detected
-----> Bootstrapping...
-----> Installing platform packages...
 !     ERROR: Failed to install system packages.

       Your platform requirements (for runtimes and extensions) could
       not be resolved to an installable set of dependencies, or a
       repository was unreachable.

       Full error information from installation attempt:

       > Loading repositories with available runtimes and extensions
       > Updating dependencies
       > Your requirements could not be resolved to an installable set of packages.
       > 
       >   Problem 1
       >     - Installation request for composer.json/composer.lock dev-15b816d355aad1a512f07471ac59a9ec -> satisfiable by composer.json/composer.lock[dev-15b816d355aad1a512f07471ac59a9ec].
       >     - composer.json/composer.lock dev-15b816d355aad1a512f07471ac59a9ec requires php 7.1.9 -> no matching package found.
       > 

       Please verify that all requirements for runtime versions in
       'composer.lock' are compatible with the list below, and ensure
       all required extensions are available for the desired runtimes.

       For reference, the following runtimes are currently available:

       PHP:  7.2.0, 7.1.12, 7.0.26, 5.6.32
       HHVM: 

       For a list of supported runtimes & extensions on Heroku, please
       refer to: https://devcenter.heroku.com/articles/php-support
 !     Push rejected, failed to compile PHP app.
 !     Push failed

Deployment to Heroku completed after PHP version updated.