Fix travis build error, old apr tarball doesn't exist anymore

```
$ wget http://ftp.jaist.ac.jp/pub/apache/apr/${APR}.tar.gz
--2018-09-21 07:16:24--  http://ftp.jaist.ac.jp/pub/apache/apr/apr-1.6.3.tar.gz
Resolving ftp.jaist.ac.jp (ftp.jaist.ac.jp)... 150.65.7.130, 2001:df0:2ed:feed::feed
Connecting to ftp.jaist.ac.jp (ftp.jaist.ac.jp)|150.65.7.130|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2018-09-21 07:16:25 ERROR 404: Not Found.
```

.travis.yml

@@ -64,7 +64,7 @@ install:
 ## ----------------------------------------------------------------------
 
 env:
-  - APR=apr-1.6.3
+  - APR=apr-1.6.5
     APR_UTIL=apr-util-1.6.1
     CC=gcc-6
     CXX=g++-6