This allows composer installs to function even if the installer checksum
changes, and doesn't force us to update the formula whenever their
installer changes.

We can do this now as composer upstream provides the checksum on
https://composer.github.io/installer.sig since 2016-04-28.

Without this installation fails with:

  The HOME or COMPOSER_HOME environment variable must be set for
  composer to install correctly

The composer installer has to be executed with PHP itself as detailed on
https://getcomposer.org/download/.