travis-ci: upgrade dist of travis-ci to ubuntu bionic. (openresty#93)

Author: xiaocang

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: trusty
+dist: bionic
 
 os: linux
 
@@ -83,4 +83,4 @@ script:
  - sh util/build.sh $NGINX_VERSION > build.log 2>&1 || (cat build.log && exit 1)
  - nginx -V
  - ldd `which nginx`|grep luajit
- - prove -r t
+ - prove -I. -r t