Install Postgis extension with yum

[a-surfer]

1. Just moved the original Debian-based Postgis dependencies from defaults/main.yml to the actual apt task in the tasks/extensions/postgis.yml file
2. Needed to make a terse variable for the postgis version in defaults/main.yml
3. Added a yum task in the tasks/extensions/postgis.yml file to install postgis with the right version

[erdenezul]

I think you could just delete lines instead of commenting those.

[taybin]

I'd love to see this merged, but agree about removing the lines instead of commenting them out.

[erdenezul]

could you please remove commented lines

[gclough]

I didn't see this @aal-ssara, @taybin, and @erdenezul ... so I wrote #310 instead. I think it goes a bit further than this one:

1. Has lookup table for PostgreSQL/PostGIS compatibility, and installs the latest compatible version
2. Moves the package names out to the vars/Blah.yml files, so it's easier to read
3. Travis does tests, and installs PostGIS during the CI validation

[aoyawale]

closing this PR since we have not received a response and there is a more later PR for this.