travis: fix skipping tagged releases

When building a PR, TRAVIS_BRANCH refers to the *target branch*.
Therefore, if a PR targets `master`, and `master` happened to be tagged,
we skipped the build by mistake.

Fix this by using TRAVIS_PULL_REQUEST_BRANCH (i.e. the *source branch*)
when available, falling back to TRAVIS_BRANCH (i.e. for CI builds, also
known as "push builds").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

ci/lib.sh

@@ -3,7 +3,7 @@
 if test true = "$TRAVIS"
 then
 	# We are running within Travis CI
-	CI_BRANCH="$TRAVIS_BRANCH"
+	CI_BRANCH="${TRAVIS_PULL_REQUEST_BRANCH:-$TRAVIS_BRANCH}"
 	CI_COMMIT="$TRAVIS_COMMIT"
 	CI_JOB_ID="$TRAVIS_JOB_ID"
 	CI_JOB_NUMBER="$TRAVIS_JOB_NUMBER"