Update erlang doc to reflect supported Erlang rels

The set of releses is now taken from the sources
specified in the erlang chef cookbook.
Update PHP docs to use `install` when appropriate

Installing dependencies with `before_script` is possible, but the `install` phase is a more appropriate one.
Updated S3 documentation.
Merge pull request #407 from Joshua-Anderson/s3-improvements

Updated S3 documentation.
Merge pull request #401 from hkdobrev/patch-1

Update PHP docs to use `install` when appropriate
Merge pull request #364 from sebastian/master

Update erlang doc to reflect supported Erlang rels
Updated email notification docs to relfect new behavior
Merge pull request #417 from Aaron1011/emails

Updated email notification docs to relfect new behavior
Update skip-ci docs to reflect the current behaviour
Update docs to reflect new Ruby versions
Merge pull request #419 from travis-ci/hh-update-rubies

Update docs to reflect new Ruby versions
Blog post on PostgreSQL 9.2 and 9.3 availability.
Merge pull request #420 from travis-ci/mm-postgresql-92-93-blog-post

Blog post on PostgreSQL 9.2 and 9.3 availability.
Add note on availability.
Update docs on secure env vars to clarify pull request availability

Secure environment variables are available if the pull request comes
from the same repository.
Add note on PostGIS.
Add blog post on our database migration.
Update with feedback fixes.
Merge pull request #421 from travis-ci/mm-blog-post-on-database-migration

Blog post on our recent database migration
Add Atom feed blog post