PHPLIB-363: Require ext-mongodb ^1.5.0 (drop support for server 2.6)

.travis.yml

@@ -15,7 +15,7 @@ cache:
 
 env:
   global:
-    - DRIVER_VERSION=1.4.0
+    - DRIVER_VERSION=1.5.0
     - SERVER_VERSION=3.6.2
 
 matrix:

composer.json

@@ -13,7 +13,7 @@
         "php": ">=5.5",
         "ext-hash": "*",
         "ext-json": "*",
-        "ext-mongodb": "^1.4.0"
+        "ext-mongodb": "^1.5.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.36 || ^6.4"