Use PHP 5.6 compatible driver release

Author: alcaeus

Diff for: .travis.yml

@@ -45,6 +45,7 @@ jobs:
     # Test against legacy driver to ensure validity of the test suite
     - stage: Test
       php: 5.6
+      env: DRIVER_VERSION="1.7.5"
       install:
         - yes '' | pecl -q install -f mongo