Fix travis build

Without lfs installed, `spec` does not end up matching `spec/*.lua` in include_files list.
Use `src` instead.

Author: mpeterv

.travis.yml

@@ -29,7 +29,7 @@ install:
 script:
   - busted -c
   - lua -e 'package.path="./src/?.lua;./src/?/init.lua;"..package.path' -lluacov bin/luacheck.lua luacheck-scm-1.rockspec -j2
-  - lua -e 'package.preload.lfs=error;package.path="./src/?.lua;./src/?/init.lua;"..package.path' -lluacov bin/luacheck.lua spec | grep 'I/O error'
+  - lua -e 'package.preload.lfs=error;package.path="./src/?.lua;./src/?/init.lua;"..package.path' -lluacov bin/luacheck.lua src | grep 'I/O error'
   - lua -e 'package.preload.lanes=error;package.path="./src/?.lua;./src/?/init.lua;"..package.path' -lluacov bin/luacheck.lua --version | grep 'Not found'
   - lua install.lua path/to/luacheck
   - mv src src2