Use custom compiled PostgreSQL on push builds
Use custom compiled PostgreSQL on push builds

Previously we compiled PostgreSQL for PR and we used already compiled
PostgreSQL packages for push builds. Compiling PostgreSQL allows us to
run isolation/vanilla tests. Thus we only ran those tests for PR builds.
We did not used compiled PostgreSQL for both builds because;
- We wanted to run our tests against packages
- Compiling takes too much time.

However, there are some benefits in using custom compiled PostgreSQL
in push builds instead of PR builds such as;
- At the moment, we do not run isolation/vanilla tests until we open a
PR, which does not make any sense.
- After merging a PR (i.e. after push to the master) push builds run
and we do not run isolation tests.
- While merging community to enterprise, we have to open a temporary
PR to make travis run isolation/vanilla tests.
- With this change master branch will have its own cache that means
every branch originated from master branch can re-use master branch's
cache. (Currently we cannot use cache for the first PR build)

With this PR we switch from using custom compiled for PR builds to push
builds.

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