travis: don't update submodules recursively

We have invalid submodules for testing and Travis gets unhappy.
diff --git a/.travis.yml b/.travis.yml
index 6131e6d..ffbba64 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,6 +11,12 @@
   allow_failures:
     - go: tip
 
+git:
+  submodules: false
+
+before_install:
+  - git submodule update --init
+
 branches:
   only:
   - master