Convert exec commands to full paths.

Without this change, I get warnings like:
```
Error: Validation of Exec[install php-5.4.24] failed: 'sudo PHPBREW_ROOT=/opt/phpbrew /usr/bin/phpbrew install --old php-5.4.24 +default +intl ' is not qualified and no path was specified. Please qualify the command or specify a path. at /tmp/vagrant-puppet-2/modules-0/phpbrew/manifests/install.pp:39
Wrapped exception:
'sudo PHPBREW_ROOT=/opt/phpbrew /usr/bin/phpbrew install --old php-5.4.24 +default +intl ' is not qualified and no path was specified. Please qualify the command or specify a path.
```