Updated .travis.yml to test multiple cache sizes

cache = 1 byte and cache = 1 block are imporant corner cases, with
cache = 16 bytes also thrown in as a more common case.

.travis.yml

@@ -1,2 +1,4 @@
 script:
-    - make test
+    - CFLAGS="-DLFS_READ_SIZE=16  -DLFS_PROG_SIZE=16"  make test
+    - CFLAGS="-DLFS_READ_SIZE=1   -DLFS_PROG_SIZE=1"   make test
+    - CFLAGS="-DLFS_READ_SIZE=512 -DLFS_PROG_SIZE=512" make test