Add testing for all supported versions of dokku

Also drop support for 0.3.x

.travis.yml

@@ -3,5 +3,6 @@ dist: trusty
 language: bash
 env:
   - DOKKU_VERSION=master
+  - DOKKU_VERSION=v0.4.14
 before_install: make setup
 script: make test

README.md

@@ -4,18 +4,13 @@ Official postgres plugin for dokku. Currently defaults to installing [postgres 9
 
 ## requirements
 
-- dokku 0.4.0+
+- dokku 0.4.x+
 - docker 1.8.x
 
 ## installation
 
 ```shell
-# on 0.3.x
-cd /var/lib/dokku/plugins
-git clone https://github.com/dokku/dokku-postgres.git postgres
-dokku plugins-install
-
-# on 0.4.x
+# on 0.4.x+
 dokku plugin:install https://github.com/dokku/dokku-postgres.git postgres
 ```