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/ParseRole.spec.js

@@ -76,7 +76,7 @@ describe('Parse Role testing', () => {
     return role.save({}, { useMasterKey: true });
   };
 
-  it_exclude_dbs(['postgres'])("should not recursively load the same role multiple times", (done) => {
+  it("should not recursively load the same role multiple times", (done) => {
     var rootRole = "RootRole";
     var roleNames = ["FooRole", "BarRole", "BazRole"];
     var allRoles = [rootRole].concat(roleNames);