Fix continuous integration tests for PHP 5.3.

Error message:
    Parse error: syntax error, unexpected T_CLASS, expecting T_STRING or T_VARIABLE or '$' in [...]

tests/before_script.sh

@@ -30,6 +30,10 @@ reload_nginx() {
     $superuser /etc/init.d/nginx restart
 }
 
+php_v5_3_shim() {
+    remove_double_colon_class_name_resolution
+}
+
 phpunit_shim() {
     # -class CurlTest extends \PHPUnit\Framework\TestCase
     # +class CurlTest extends \PHPUnit_Framework_TestCase
@@ -51,6 +55,10 @@ phpunit_shim() {
     sed -i'' -e"s/${find}/${replace}/" "$(pwd)/tests/PHPCurlClass/PHP"*
 }
 
+remove_double_colon_class_name_resolution() {
+    sed -i'' -e"/::class/d" "$(pwd)/tests/PHPCurlClass/PHP"*
+}
+
 remove_expectWarning() {
     # Fix "Call to undefined method CurlTest\CurlTest::expectWarning()".
     sed -i'' -e"/->expectWarning(/d" "$(pwd)/tests/PHPCurlClass/PHP"*
@@ -129,6 +137,7 @@ EOF
     $superuser /etc/init.d/php5-fpm start
     reload_nginx
     phpunit_shim
+    php_v5_3_shim
 elif [[ "${TRAVIS_PHP_VERSION}" == "5.4" ]]; then
     install_nginx
     use_php_fpm