fix travis

Author: chambart

.travis.yml

@@ -3,10 +3,13 @@ sudo: required
 install: wget https://raw.githubusercontent.com/ocaml/ocaml-ci-scripts/master/.travis-opam.sh
 script: bash -ex .travis-opam.sh
 env:
-  - OCAML_VERSION=4.02 TESTS=true
-  - OCAML_VERSION=4.03 TESTS=true
-  - OCAML_VERSION=4.04 TESTS=true
-  - OCAML_VERSION=4.05 TESTS=true
+  global:
+    - PACKAGE=nullable-array
+    - TESTS=true
+  matrix:
+    - OCAML_VERSION=4.02
+    - OCAML_VERSION=4.03
+    - OCAML_VERSION=4.04
+    - OCAML_VERSION=4.05
 os:
   - linux
-  - osx