Add missing include.

.travis.yml

@@ -63,6 +63,7 @@ addons:
 
   homebrew:
     taps: homebrew/science
+    update: true
     packages:
       - git
       - cmake

test/collision.cpp

@@ -44,6 +44,7 @@
 #include <boost/test/unit_test.hpp>
 #include <boost/utility/binary.hpp>
 
+#include <fstream>
 #include <boost/filesystem.hpp>
 #include <boost/assign/list_of.hpp>