travis-ci. set ubuntu version to trusty

From Tuesday April 23rd, 2019, Travis-ci started to use Ubuntu Xenial as
default distro[1], this introduce new mysql version 5.7.x[2] and incompatible
datetime format[3].

[1] https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
[2] https://docs.travis-ci.com/user/database-setup/#mysql
[3] go-sql-driver/mysql#725

Author: Amygos

.travis.yml

@@ -1,5 +1,7 @@
 if: tag != autobuild
 
+dist: trusty
+
 matrix:
   include:
     - language: go