travis-ci: upgrade dist of travis-ci to ubuntu bionic. (#66)

Author: xiaocang

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: trusty
+dist: bionic
 
 os: linux
 
@@ -57,5 +57,5 @@ script:
   - cd ..
   - export PATH=$PWD/work/nginx/sbin:$PWD/nginx-devel-utils:$PATH
   - ngx-build $NGINX_VERSION --with-ipv6 --with-http_realip_module --with-http_ssl_module --add-module=../lua-nginx-module --with-debug --with-cc-opt="-I$OPENSSL_INC" --with-ld-opt="-L$OPENSSL_LIB -Wl,-rpath,$OPENSSL_LIB" > build.log 2>&1 || (cat build.log && exit 1)
-  - prove -r t
+  - prove -I. -r t