Add script to upgrade bundler to Gemfile version

Useful to sync your bundler version or the CI bundler version with the
rest of the team.

script/upgrade_bundler.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+# This shell script looks for the last used Bundler version in Gemfile.lock and
+# installs exactly that version, removing other versions.
+
+# Fail if a single command fails.
+set -e
+
+# `grep -m 1`: find the first occurrence of "BUNDLED WITH"
+# `-A`:        print the next line after "BUNDLED WITH" as well
+# `-x -F`:     find exactly that string without interpreting regex
+# `tail -n 1`: take the last line, the version line
+# `tr -d`:     delete all spaces, the indent before the version
+version="$(grep -m 1 -A 1 -x -F "BUNDLED WITH" Gemfile.lock | tail -n 1  | tr -d '[:space:]')"
+
+if [ -z "$version" ]; then
+  echo "No bundler version in Gemfile.lock."
+  exit 1
+fi
+
+current="$(bundler --version)"
+
+if [ "$current" = "Bundler version $version" ]; then
+  echo "Already up-to-date: $current"
+  exit 0
+fi
+
+gem install bundler -v "$version"
+gem cleanup bundler