Fixes and local tests

Hello,

So these are a set of fixes and tests that I've happened to perform whilst adding a feature for the role (said feature is still WIP but basically I want to be able to manage postgres users).

Resuming this PR:
- Fixed travis builds, which we're failing because travis has already several (9.1, 9.2 and 9.3) versions of postgres installed. Simply removing them wouldn't work because we need to remove some but not all of the config files that are contained in the postgres packages.
- Fixed idempotency in some tasks (more info in the git commit log);
- Now using version_compare in templates/master.conf.j2 for our version specific configs' checks.
- Added Vagrant-based local tests;
- Added specific tags per set of tasks (you can keep the "postgres" tag in all tasks by adding it when you include the role in your playbooks, by adding "tags: postgres". 

Thank you!