Set Travis clone depth to 1, to alleviate workload

This should make Travis only test the last commit of every PR.
diff --git a/.travis.yml b/.travis.yml
index 0e10a83..4cdad37 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,5 @@
+git:
+  depth: 1
 language: objective-c
 osx_image: xcode7.3
 env: