travis-ci: updated ftp.pcre.org to sourceforce mirror. (#279)

Author: xiaocang

.travis.yml

@@ -49,7 +49,7 @@ services:
 install:
   - sudo apt update
   - sudo apt install --only-upgrade ca-certificates
-  - if [ ! -f download-cache/pcre-$PCRE_VER.tar.gz ]; then wget -P download-cache/ https://ftp.pcre.org/pub/pcre/pcre-$PCRE_VER.tar.gz; fi
+  - if [ ! -f download-cache/pcre-$PCRE_VER.tar.gz ]; then wget -P download-cache/ https://downloads.sourceforge.net/project/pcre/pcre/${PCRE_VER}/pcre-${PCRE_VER}.tar.gz; fi
   - if [ ! -f download-cache/openssl-$OPENSSL_VER.tar.gz ]; then wget -P download-cache https://www.openssl.org/source/openssl-$OPENSSL_VER.tar.gz || wget -P download-cache https://www.openssl.org/source/old/${OPENSSL_VER//[a-z]/}/openssl-$OPENSSL_VER.tar.gz; fi
   - git clone https://github.com/openresty/openresty-devel-utils.git
   - git clone https://github.com/openresty/lua-cjson.git