Use cached PostgreSQL build to reduce testing time

With this PR, we started to cache custom compiled PostgreSQL builds. If there
are no new commits to the related PostgreSQL branches, we will use already
compiled binaries to reduce testing time.

.travis.yml

@@ -1,7 +1,10 @@
 sudo: required
 dist: trusty
 language: c
-cache: apt
+cache:
+  apt: true
+  directories:
+  - /home/travis/postgresql
 branches:
   except: [ /^open-.*$/ ]
 env:
@@ -14,7 +17,7 @@ env:
     - PGVERSION=9.5
     - PGVERSION=9.6
 before_install:
-  - git clone -b v0.6.1 --depth 1 https://github.com/citusdata/tools.git
+  - git clone -b use_cached_postgresql_build --depth 1 https://github.com/citusdata/tools.git
   - sudo make -C tools install
   - setup_apt
   - curl https://install.citusdata.com/community/deb.sh | sudo bash