Makefile: rework bootstrap

`gem install bundler` only needs to be run if `vendor` doesn't exist.
`bundle install` should be run whenever `Gemfile` OR `Gemfile.lock`
changes.

Author: benesch

Makefile

@@ -64,7 +64,9 @@ test: bootstrap
 	go get -u github.com/cockroachdb/htmltest
 	htmltest
 
-bootstrap: Gemfile.lock
+vendor:
 	gem install bundler
+
+bootstrap: Gemfile Gemfile.lock vendor
 	bundle install
 	touch $@

ci/publish

@@ -12,5 +12,6 @@ run() {
 
 # Tell the Makefile we've already installed dependencies.
 touch "$root/../bootstrap"
+touch "$root/../vendor"
 
 run make standard-build managed-build

ci/pull-request

@@ -12,6 +12,7 @@ baseurl=$BUILD_VCS_NUMBER
 
 # Tell the Makefile we've already installed dependencies.
 touch bootstrap
+touch vendor
 
 # Step 1. Build the standard docs.
 cat > _config_prod.yml <<EOF
@@ -39,7 +40,7 @@ COMMENT_BODY=$(
 git diff --name-status $(git merge-base HEAD origin/master) | \
 awk -v "base_url=http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/$BUILD_VCS_NUMBER/" '
 BEGIN {
-  print "Online preview: " base_url 
+  print "Online preview: " base_url
 }
 
 /^[AM]\s+[^_].*\.(md|html)/ {