Enable a few tests to test against Postgres (#2971)

* Enable RestCreate related tests for Postgres

* Enable tests in ParseRole for postgres

* Enable one ParseObject test for postgres

* Improve the way a jsonb field is updated.

* Use Postgres 9.5 in Travis

* Install Postgres 9.5 in Travis

* Use sudo to install and setup Postgres 9.5 in Travis

* Test if Postgres 9.5 can be used with trusty

* Re-exclude a test for postgres

* Revert travis to its original form

spec/ParseObject.spec.js

@@ -1066,7 +1066,7 @@ describe('Parse.Object testing', () => {
     });
   });
 
-  it_exclude_dbs(['postgres'])("saving children in an array", function(done) {
+  it("saving children in an array", function(done) {
     var Parent = Parse.Object.extend("Parent");
     var Child = Parse.Object.extend("Child");