Fix syntax error on PHP 5.4
Fix syntax error on PHP 5.4

Since PHP 5.4 is still used in [Debian Wheezy][1], which is on LTS until
[May 2018][2], support for older versions of PHP would be nice.
Fortunately, the library is still compatible with PHP 5.4, except
for one small detail.

Use of `empty` on `Phar::running` was replaced by `=== ''` check, since
the function only [returns a string][3] and calling `empty` directly
on the return of function call is not supported [prior PHP 5.5][4].

[1]: https://wiki.debian.org/PHP#Available_versions
[2]: https://wiki.debian.org/LTS
[3]: https://secure.php.net/manual/en/phar.running.php#refsect1-phar.running-returnvalues
[4]: https://secure.php.net/manual/en/function.empty.php#refsect1-function.empty-parameters

abraham/twitteroauth,shakaran/twitteroauth