Travis: fix mongodb key expired error

[0xc0170]

Description

Fix for "The following signatures were invalid: KEYEXPIRED 1515625755" failed".

See travis-ci/travis-ci#9037

A lot of reports dates back to January or last year. I noticed them mainly these days. Latest build for instance with failure: https://travis-ci.org/ARMmbed/mbed-os/jobs/394488598

This should fix it as reported.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

Travis is green 💯

[cmonr]

@0xc0170 Don't hate me, but I think I'm going to cycle Travis CI on this PR a couple of times before OKing the PR.

[cmonr]

👍

Passed Travis CI 4x times without a hitch!

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2404
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7274/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2039
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7274/

[mbed-ci]

Test : SUCCESS

Build number : 2190
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7274/2190