CI: Change --branch to v3.8 and remove dist

.travis.yml

@@ -1,9 +1,8 @@
-dist: trusty
 sudo: required
 language: minimal
 script:
   - sudo ./alpine-make-rootfs
-        --branch v3.7
+        --branch v3.8
         --packages 'ruby ruby-bigdecimal sqlite'
         --script-chroot
         example-$(date +%Y%m%d).tar.gz -- ./example/install.sh