Changed littlefs-fuse target for testing purposes

This is a downside caused by relying on and external repo for testing,
but also storing the CI configuration inside this repo. Fortunately we
can use a temporary v2-alpha branch in the FUSE repo mirroring the
v2-alpha branch for testing.

.travis.yml

@@ -103,7 +103,7 @@ jobs:
         - NAME=littlefs-fuse
       install:
         - sudo apt-get install libfuse-dev
-        - git clone --depth 1 https://github.com/geky/littlefs-fuse
+        - git clone --depth 1 https://github.com/geky/littlefs-fuse -b v2-alpha
         - fusermount -V
         - gcc --version
       before_script: