travis: 4.07.1 is ambiguous on MacOS b/c ocaml-system.4.07.1 is avail…

…able, which makes opam abort.

[ERROR] Compiler selection '4.07.1' is ambiguous. matching packages: {
        ocaml-base-compiler.4.07.1, ocaml-system.4.07.1 }

On Linux we don't have any OCaml installed, so safe choice is to always download and compile OCaml by specifying ocaml-base-compiler.4.07.1.

make.sh

@@ -17,7 +17,7 @@ ocb() {
 opam_setup() {
   set -x
   opam init -y -a --bare $SANDBOXING # sandboxing is disabled in travis and docker
-  opam switch -y create ./ --deps-only 4.07.1
+  opam switch -y create ./ --deps-only ocaml-base-compiler.4.07.1
   # opam install camlp4 mongo # camlp4 needed for mongo
 }