Fixes do/release being broken with newer versions of git

Because `head` exits from `git tag` after the first line is passed
when `git` tries to push additional lines into the pipe it will receive
a `SIGPIPE` for a broken pipe and exit with a nonzero exit code, which
causes the entire script to fail due to `-e` and `-o pipefail`.

The simple solution is to reverse the sort order and use `tail` instead.

do/release

@@ -106,7 +106,7 @@ function configure-sentry() {
 }
 
 function current-version() {
-    git tag --sort=-version:refname --merged master | head -n 1
+    git tag --sort=version:refname --merged master | tail -n 1
 }
 
 function announce-release-on-sentry-io() {