fix `cache_path: command not found` error

- the following error occurred when `always_rebuild` is true
       Cleaning all cache to force rebuilds
/tmp/buildpack_xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/lib/misc_funcs.sh:
line 92: cache_path: command not found
Merge pull request #17 from ma2gedev/cache_path_error

fix `cache_path: command not found` error