Added shell function to retrieve the current PHP version

I was trying to embed current php version info into my zsh shell prompt using the `current_php_version` shell function (as mentioned in [README.md](https://github.com/phpbrew/phpbrew#enabling-version-info-prompt)) but I discovered that the function is not defined in `.phpbrew/bashrc` file so I created a simple shell function that implements this functionality along with some code duplication fixes in `/src/PhpBrew/VariantBuilder.php`.

I have also created a simple [.oh-my-zsh plugin](https://github.com/robbyrussell/oh-my-zsh/pull/3104) that provides some aliases to common phpbrew subcommands.