Merge branch 'cache-apt-and-ruby-only' into 'master'

Cache only apt and ruby from vendor

## What does this MR do?

Optimises caching mechanism when testing gitlab-ce/ee repositories.


See merge request !4619

.gitlab-ci.yml

@@ -7,7 +7,8 @@ services:
 cache:
   key: "ruby21"
   paths:
-  - vendor
+  - vendor/apt
+  - vendor/ruby
 
 variables:
   MYSQL_ALLOW_EMPTY_PASSWORD: "1"

scripts/prepare_build.sh

@@ -16,10 +16,10 @@ retry() {
 }
 
 if [ -f /.dockerenv ] || [ -f ./dockerinit ]; then
-    mkdir -p vendor
+    mkdir -p vendor/apt
 
     # Install phantomjs package
-    pushd vendor
+    pushd vendor/apt
     if [ ! -e phantomjs_1.9.8-0jessie_amd64.deb ]; then
         wget -q https://gitlab.com/axil/phantomjs-debian/raw/master/phantomjs_1.9.8-0jessie_amd64.deb
     fi