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

minor improvements to archlinux php cli.

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

This comit is in addition to pull request #70. The add-apt-repository command
will only be run when the ppa has not yet been added.

In order to reuse the default structure of this formula
I changed the config a little bit to configure hhvm files.

Signed-off-by: René Jochum <rene@jochums.at>

XML module is already built-in. In any case, `php5-xmlrpc` package actually provides XML-RPC module.

See README.rst for more information