minor: pin travis to mongoc 1.3.4

Author: Matt Cotter

.travis.yml

@@ -40,7 +40,7 @@ before_install:
 
     # Mongo C Driver
     # TODO switch master (branch for current 1.3 dev) to r1.3 when released
-    - git clone -b master https://github.com/mongodb/mongo-c-driver.git
+    - git clone -b '1.3.4' --single-branch https://github.com/mongodb/mongo-c-driver.git
 
 install:
     # GCC