Update pkgver in the PKGBUILD for v25

Add a post_upgrade message for the changes since v24.
Replace backticks with more preferred command substitution syntax in the
.install file
PKGBUILD-git: Fix possible vercmp issue with pkgver() function

This inserts the number of commits between the date and the commit hash.

This should resolve an issue that could occur when building the package
twice in one day when multiple commits were made on the same day where
the latter commit(s)' hash would trick vercmp into thinking a package
downgrade was happening.
PKGBUILDs: Add 'gem2arch' as a dependency
Merge pull request #54 from jrstjohn/post-v25-fixes

Post-v25 fixes