travis-ci: upgrade dist of travis-ci to ubuntu bionic. (#8)

Author: Johnny Wang

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: trusty
+dist: bionic
 
 os: linux
 
@@ -61,4 +61,4 @@ script:
   - export NGX_BUILD_CC=$CC
   - sh util/build.sh $NGINX_VERSION > build.log 2>&1 || (cat build.log && exit 1)
   - nginx -V
-  - prove -r t
+  - prove -I. -r t