littlefs: Set specific hash for littlefs-fuse testing in Travis

[geky]

The self-hosted littlefs-fuse testing in Mbed OS is currently dependent on the master of littlefs-fuse, which is a moving target. Locking this to a specific hash will remove this problem. Other options could be to commit the littlefs-fuse wrapper into mbed-os specifically for testing, or remove fuse testing.

see #5903 (comment)
cc @theotherjimmy, @0xc0170

Note: I don't think device testing is needed

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 955
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5928/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 640
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5928/

[geky]

Uh... how? what?

[kegilbert]

@geky Java exception on a Windows test node....need to restart.
/morph export-build

[mbed-ci]

Test : SUCCESS

Build number : 783
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5928/783

[mbed-ci]

Exporter Build : SUCCESS

Build number : 649
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5928/