Update Zopfli and several other depenencies (shssoichiro#512)

* Update Zopfli and several other depenencies

I've just published a new Zopfli release, v0.7.3, which includes several
new features and internal refactors. Performance should be a tad bit
better, but I didn't test it throughly, so YMMV. Perhaps more
importantly for OxiPNG, its dependency tree is smaller, and
Gzip-exclusive compression code can be excluded at compile time thanks
to new feature switches. As I mentioned on
shssoichiro#495 (comment),
I plan on delivering more significant Zopfli performance improvements at
some point, but for now I think it's good to give the new release more
real-world usage and testing 😄

While at it, I've upgraded other dependencies that are not
performance-critical to their latest semver-compatible versions. This
excludes `libdeflater` on purpose, as its performance characteristics
are said to be somewhat different.

* Update Zopfli to v0.7.4

v0.7.3 was superseeded shortly after v0.7.3 was released to address a
last minute change to the new API it introduced. OxiPNG is not affected
by this, but I think it's good practice to update Zopfli anyway.