Revise Travis build matrix

Remove MongoDB 2.4 from the build matrix and use the latest stable driver by default.

Add explicit jobs to test MongoDB 2.4, driver 1.1.0 (oldest supported version), and the latest unstable driver version. Lastly, restrict PHP 5.4 and 5.5 to a single job each.

.travis.yml

@@ -13,10 +13,27 @@ env:
     - MONGO_REPO_TYPE="precise/mongodb-enterprise/"
     - SOURCES_LOC="/etc/apt/sources.list.d/mongodb.list"
   matrix:
-    - DRIVER_VERSION=1.1.2 SERVER_VERSION=2.4
-    - DRIVER_VERSION=1.1.2 SERVER_VERSION=2.6
-    - DRIVER_VERSION=1.1.2 SERVER_VERSION=3.0
-    - DRIVER_VERSION=1.1.2 SERVER_VERSION=3.2
+    - DRIVER_VERSION=stable SERVER_VERSION=2.6
+    - DRIVER_VERSION=stable SERVER_VERSION=3.0
+    - DRIVER_VERSION=stable SERVER_VERSION=3.2
+
+matrix:
+  include:
+    - php: 5.6
+      env: DRIVER_VERSION=1.1.0 SERVER_VERSION=3.2
+    - php: 7.0
+      env: DRIVER_VERSION=stable SERVER_VERSION=2.4
+    - php: 7.0
+      env: DRIVER_VERSION=devel SERVER_VERSION=3.2
+  exclude:
+    - php: 5.4
+      env: DRIVER_VERSION=stable SERVER_VERSION=2.6
+    - php: 5.4
+      env: DRIVER_VERSION=stable SERVER_VERSION=3.0
+    - php: 5.5
+      env: DRIVER_VERSION=stable SERVER_VERSION=2.6
+    - php: 5.5
+      env: DRIVER_VERSION=stable SERVER_VERSION=3.0
 
 before_install:
   - sudo apt-key adv --keyserver ${KEY_SERVER} --recv 7F0CEB10