Use the new apt addon to install libevent-dev.

hannosch · hannosch · commit 3a6e37aa8f42 · 2015-05-28T11:55:20.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,10 @@
 sudo: false
 
+addons:
+  apt:
+    packages:
+    - libevent-dev
+
 cache:
   directories:
     - $HOME/.cache/pip
