Use OCaml 4.07

Signed-off-by: Edwin Török <[email protected]>

Author: edwintorok

.travis.yml

@@ -9,4 +9,4 @@ env:
     - PINS="xapi-networkd:."
     - BASE_REMOTE="https://github.com/xapi-project/xs-opam.git"
   matrix:
-    - DISTRO="debian-9-ocaml-4.06"
+    - DISTRO="debian-9-ocaml-4.07"