tests: change typo memcache in .travis.yml. (#52)

Author: zhuizhuhaomeng

.travis.yml

@@ -21,6 +21,7 @@ addons:
     - luarocks
     - iproute2
     - bc
+    - memcached
 
 cache:
   directories:
@@ -50,7 +51,7 @@ env:
     - NGINX_VERSION=1.19.3 OPENSSL_VER=1.1.1i OPENSSL_PATCH_VER=1.1.1f
 
 services:
-  - memcache
+  - memcached
 
 before_install:
   - sudo luarocks install luacheck $LUACHECK_VER