travis-ci: upgrade dist of travis-ci to ubuntu bionic. (openresty#60)

Author: xiaocang

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: trusty
+dist: bionic
 
 os: linux
 
@@ -65,4 +65,4 @@ script:
  - ngx-build $NGINX_VERSION --with-ipv6 --with-http_realip_module --with-http_ssl_module --with-cc-opt="-I$OPENSSL_INC" --with-ld-opt="-L$OPENSSL_LIB -Wl,-rpath,$OPENSSL_LIB" --add-module=../ndk-nginx-module --add-module=../lua-nginx-module --with-debug > build.log 2>&1 || (cat build.log && exit 1)
  - nginx -V
  - ldd `which nginx`|grep -E 'luajit|ssl|pcre'
- - prove -r t
+ - prove -I. -r t