Fixed migration test condition for prefix branches

Both the littlefs-fuse and littlefs-migration test jobs depend on
the external littlefs-fuse repo. But unfortunately, the automatic
patching to update the external repo with the version under test
does not work with the prefix branches.

In this case we can just skip these tests, they've already been tested
multiple times to get to this point.

.travis.yml

@@ -136,6 +136,7 @@ jobs:
       env:
         - STAGE=test
         - NAME=littlefs-migration
+      if: branch !~ -prefix$
       install:
         - sudo apt-get install libfuse-dev
         - git clone --depth 1 https://github.com/geky/littlefs-fuse -b v2-alpha v2