fix LDAP install for systems without ldap extension (#846)

composer.json

@@ -60,8 +60,7 @@
         "symfony/webpack-encore-bundle": "^1.5",
         "symfony/yaml": "^4.0",
         "twig/extensions": "^1.5",
-        "white-october/pagerfanta-bundle": "^1.1",
-        "zendframework/zend-ldap": "^2.10"
+        "white-october/pagerfanta-bundle": "^1.1"
     },
     "require-dev": {
         "dama/doctrine-test-bundle": "^5.0",
@@ -100,7 +99,8 @@
         "symfony/polyfill-php56": "*"
     },
     "suggest": {
-        "ext-mbstring": "If ext-mbstring is not available you MUST install symfony/polyfill-mbstring"
+        "ext-mbstring": "If ext-mbstring is not available you MUST install symfony/polyfill-mbstring",
+        "zendframework/zend-ldap": "For LDAP authentication with Kimai"
     },
     "scripts": {
         "auto-scripts": {