Specify distro in travis to ubuntu:trusty(14.04)

unasuke · unasuke · commit afecfdc67d5a · 2017-01-07T01:25:45.000+09:00
for use rubinius 2.x
diff --git a/.travis.yml b/.travis.yml
@@ -1,3 +1,5 @@
+dist: trusty
+
 rvm:
   - 2.1.10
   - 2.2.6
