Add mbstring dependency to appveyor configuration

Author: acoburn

appveyor.yml

@@ -15,6 +15,7 @@ install:
     - echo date.timezone="UTC" >> php.ini
     - echo extension_dir=ext >> php.ini
     - echo extension=php_openssl.dll >> php.ini
+    - echo extension=php_mbstring.dll >> php.ini
     - SET PATH=C:\tools\php71;%PATH%
     - cd c:\projects\static-ldp
     - php -r "readfile('http://getcomposer.org/installer');" | php