Download astyle from AWS instead of SourceForge

[cmonr]

SF has been failing intermitantly, and appears to not explicitly allow the automatic downloading of archives.

Description

Example of the success: https://travis-ci.org/cmonr/mbed-os/jobs/396958157

Pull request type

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

[cmonr]

Going to start CI since this is has passed Travis twice now.
/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2451
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7338/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2081
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7338/

[mbed-ci]

Test : FAILURE

Build number : 2229
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7338/2229

[mbed-ci]

Test : SUCCESS

Build number : 2230
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7338/2230