Add ext-mbstring to requirements

`mb_strpos` function is used in several places in the project. Many features will not work without `ext-mbstring` installed.

Author: alexmnv

composer.json

@@ -15,7 +15,8 @@
     "php": "^5.6 || ~7.0",
     "symfony/process": "^2.8 || ^3.1 || ^4.0",
     "ext-curl": "*",
-    "ext-zip": "*"
+    "ext-zip": "*",
+    "ext-mbstring": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "^5.7",