Allow tests to run without using the mbstring extension

This really is a workaround for this issue:
travis-ci/travis-ci#4701

There is currently no way to disable PHP extensions on Travis CI, so we have to conditionally use them in the code itself.

.travis.yml

@@ -6,6 +6,10 @@ php:
   - hhvm
   - nightly
 
+env:
+  - USE_PHP_EXTENSIONS=FALSE
+  - USE_PHP_EXTENSIONS=TRUE
+
 matrix:
   allow_failures:
     - php: hhvm

phpunit.php

@@ -0,0 +1,14 @@
+<?php
+
+require __DIR__ . '/vendor/autoload.php';
+
+echo 'Using PHP extensions: ';
+
+if (getenv('USE_PHP_EXTENSIONS') === 'FALSE') {
+    define('NO_PHP_EXTENSIONS', true);
+    echo 'NO';
+} else {
+    echo 'YES';
+}
+
+echo PHP_EOL;

phpunit.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true" bootstrap="vendor/autoload.php">
+<phpunit colors="true" bootstrap="phpunit.php">
     <testsuites>
         <testsuite>
             <directory>tests</directory>

src/Validator/StringValidator.php

@@ -72,7 +72,7 @@ protected function validate(string $value) : void
      */
     private function getLength(string $string) : ?int
     {
-        if (extension_loaded('mbstring')) {
+        if (extension_loaded('mbstring') && ! defined('NO_PHP_EXTENSIONS')) { // constant used for tests
             if (! mb_check_encoding($string, 'UTF-8')) {
                 return null;
             }