Add ninja build.
diff --git a/.travis.yml b/.travis.yml
index 3aab8f5..a7544a1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,11 @@
   directories:
     - $HOME/cpu_features_archives
 
+addons:
+  apt:
+    packages:
+    - ninja-build
+
 matrix:
   include:
   - os: linux