adapting to new s3 bucket location in travis.yml

[saheerb]

Summary of changes

Travis CI uses compilers and other binaries like astyle. These are stored in S3 bucket currently owned by ISG and mbed-os-ci is switching to OSS. This PR adapts the S3 location changes from ISG to OSS for travis CI builds.

Impact of changes

Migration actions required

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@saheerb, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[saheerb]

@adbridge @jamesbeyond please review

[adbridge]

@saheerb please fill in the template header correctly, there is a lot of missing, required information...