Fix Travis CI build

I needed to make the following changes to fix the Travis build:

- Add missing fields to the opam file

- Use the make variable in the opam file instead of the "make" command -
  this change is necessary for the "opam lint" check pass.

- Use a version of cow package that has cow.syntax - Cow 2.0.0 has
  removed camlp4 syntax extension support:
  https://github.com/mirage/ocaml-cow/blob/master/CHANGES.md

- Remove invalid flake8 option

Signed-off-by: Gabor Igloi <gabor.igloi@citrix.com>

.travis.yml

@@ -7,7 +7,7 @@ install:
   - wget https://raw.githubusercontent.com/ocaml/ocaml-travisci-skeleton/master/.travis-opam.sh
   - pip install --user flake8
 script:
-  - flake8 --show-source --show-pep8 python
+  - flake8 --show-source python
   - bash -ex .travis-opam.sh
 env:
   - OCAML_VERSION=4.02 PACKAGE=xapi-storage

opam

@@ -1,14 +1,20 @@
-opam-version: "1"
+opam-version: "1.2"
 maintainer: "dave.scott@citrix.com"
+authors: "xen-api@lists.xen.org"
+homepage: "https://xapi-project.github.io/"
+bug-reports: "https://github.com/xapi-project/xapi-storage/issues"
+dev-repo: "git://github.com/xapi-project/xapi-storage.git"
 build: [
   [make]
+]
+install: [
   [make "install" "PREFIX=%{prefix}%"]
 ]
 remove: [
-  ["make" "uninstall" "PREFIX=%{prefix}%"]
+  [make "uninstall" "PREFIX=%{prefix}%"]
 ]
 depends: [
-  "cow"
+  "cow" {< "2.0.0"}
   "rpc"
   "caml2html"
   "xmlm"