Update Travis to use ext-mongodb 1.4.0RC1

This removes the redundant "devel" matrix entry and refactors the config to inherit/override global environment vars.

.travis.yml

@@ -10,54 +10,55 @@ addons:
     packages: &common_packages
       - gdb
 
+env:
+  global:
+    - DRIVER_VERSION=1.4.0RC1
+    - SERVER_VERSION=3.4
+
 matrix:
   fast_finish: true
   include:
     - php: 5.5
-      env: &common_env DRIVER_VERSION=1.3.3 SERVER_VERSION=3.4
       addons: &common_addons
         apt:
           sources: [ mongodb-3.4-precise ]
           packages: [ mongodb-org, *common_packages ]
     - php: 5.6
-      env: *common_env
       addons: *common_addons
     - php: 7.0
-      env: *common_env
       addons: *common_addons
     - php: 7.1
-      env: *common_env
       addons: *common_addons
     - php: 7.2
-      env: *common_env
       addons: *common_addons
     - php: 7.0
-      env: DRIVER_VERSION=1.3.3 SERVER_VERSION=2.4
+      env:
+        - SERVER_VERSION=2.4
       addons:
         apt:
           sources: [ mongodb-upstart ]
           packages: [ mongodb-10gen, *common_packages ]
     - php: 7.0
-      env: DRIVER_VERSION=1.3.3 SERVER_VERSION=2.6
+      env:
+        - SERVER_VERSION=2.6
       addons:
         apt:
           sources: [ mongodb-upstart ]
           packages: [ mongodb-org, *common_packages ]
     - php: 7.0
-      env: DRIVER_VERSION=1.3.3 SERVER_VERSION=3.0
+      env:
+        - SERVER_VERSION=3.0
       addons:
         apt:
           sources: [ mongodb-3.0-precise ]
           packages: [ mongodb-org, *common_packages ]
     - php: 7.0
-      env: DRIVER_VERSION=1.3.3 SERVER_VERSION=3.2
+      env:
+        - SERVER_VERSION=3.2
       addons:
         apt:
           sources: [ mongodb-3.2-precise ]
           packages: [ mongodb-org, *common_packages ]
-    - php: 7.0
-      env: DRIVER_VERSION=devel SERVER_VERSION=3.4
-      addons: *common_addons
 
 before_script:
   - mongod --version