.travis.yml: Add explicit manifest to luarocks invocation

Author: daurnimator

.travis.yml

@@ -24,7 +24,7 @@ before_install:
   - luarocks install busted
 
 install:
-  - luarocks install --only-deps spawn-scm-0.rockspec
+  - luarocks install --only-deps --server=http://luarocks.org/manifests/daurnimator spawn-scm-0.rockspec
 
 script:
   - luacheck .