Adapt travis script to use the examples development branch

.travis/build_examples.sh

@@ -3,11 +3,19 @@ repo_path="$HOME/projects/unipdf-examples"
 git clone https://github.com/unidoc/unipdf-examples.git $repo_path
 cd $repo_path
 
-branch_name="v3"
-if [[ `git ls-remote origin "$TRAVIS_BRANCH"` ]]; then
-    branch_name="$TRAVIS_BRANCH"
+build_branch="$TRAVIS_BRANCH"
+if [[ -n "$TRAVIS_PULL_REQUEST_BRANCH" ]]; then
+    build_branch="$TRAVIS_PULL_REQUEST_BRANCH"
+fi
+if [[ -z "$build_branch" ]]; then
+    build_branch="development"
+fi
+
+target_branch="development"
+if [[ `git ls-remote origin "$build_branch"` ]]; then
+    target_branch="$build_branch"
 fi
 
-git checkout $branch_name
+git checkout $target_branch
 echo "replace github.com/unidoc/unipdf/v3 => $TRAVIS_BUILD_DIR" >> go.mod
 ./build_examples.sh