The "-k" option tells gzip to keep the original files intact and is an
alternative to `gzip -c file > file.gz`. It was implemented in 2013:
http://git.savannah.gnu.org/cgit/gzip.git/commit/?id=0192f02
There are a few popular operating systems (ie, Red Hat 7, Debian Wheezy)
whose version of gzip does not have the "-k" option. Compiling assets
breaks on these operating systems. Using "-c" instead ensures that it
works even with older versions of gzip.