Have Travis run mobile tests that use the parent code

[hypest]

Description

Until now, Travis was running the mobile tests but those were actually using the deeply nested Gutenberg filetree (a nested submodule inside the mobile submodule), resulting to false positive "green" runs.

This PR adds a flag to the mobile related Travis jobs to make them use the proper configuration where the mobile tests should assume to be running from inside the Gutenberg folder.

How has this been tested?

This is a Travis change and the testing is done by looking at the Travis run on this PR.
In particular, when looking at the 2 mobile job outputs, one can see this:

$ ./.travis/travis-checks-js.sh
> gutenberg-mobile@0.1.0 test:inside-gb /home/travis/build/WordPress/gutenberg/gutenberg-mobile
> cross-env NODE_ENV=test node node_modules/jest/bin/jest.js --verbose --config jest_gb.config.js

where the important bit is the jest_gb.config.js. That's the Jest configuration that sets the Gutenberg package paths to point one directory up from the mobile submodule. That's what we need to have the mobile tests use the code/changes found in the parent folder.

Types of changes

Adds a shell flag to the Travis configuration of the mobile jobs.

To test:

Open the Travis output https://travis-ci.org/WordPress/gutenberg/jobs/430595120#L5410 and notice the jest_gb.config.js bit.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[hypest]

Headsup , the 2 Travis mobile jobs failing right now beacuse current Gutenberg master (32a9c1a at the time of writing) has npm test failing on the mobile tests.

Edit: fixed with fb8cc59