Explicity #close each zip member file that has been opened via #fopen.

Addresses a memory leak with the zipruby gem where using `fopen()` and `#read` without explicitly closing the file afterward will never release the memory used reading the file. This pull request uses `#fopen()` with a block so that zipruby will close the file after reading and free the memory.

I prefer braces for block whose return value will be assigned to a variable.
Can you rewrite this?
(I know I don't do that at other lines written by me. Sorry for that. I'll fix them later.)