Reduce size of flags

The PageSpeed tool of Google Webmaster tools signals that
"a lossless compression can reduce the file size to reduce page load time"

Let's do that: I reduced the size of the flags with this command line:
pngcrush -brute -force -reduce

The size of the images lowered from 327.367 to 233.841 bytes (28% gain).
Merge pull request #53 from mlocati/reduce-flags-filesize

Reduce size of flags