Update .travis.yml after move to ocaml 4.06

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

.travis.yml

@@ -9,14 +9,14 @@ script: bash -ex ./.travis-docker.sh
 env:
   global:
     - PINS="stdext:. xapi-stdext:. xapi-stdext-base64:. xapi-stdext-bigbuffer:. xapi-stdext-date:. xapi-stdext-deprecated:. xapi-stdext-encodings:. xapi-stdext-monadic:. xapi-stdext-pervasives:. xapi-stdext-range:. xapi-stdext-std:. xapi-stdext-threads:. xapi-stdext-unix:. xapi-stdext-zerocheck:."
+    - OCAML_VERSION=4.06.0
     - DISTRO="ubuntu-16.04"
     - TEST=false
     - BASE_REMOTE="https://github.com/xapi-project/xs-opam"
   matrix:
-    - PACKAGE=xapi-stdext OCAML_VERSION=4.04.2 \
+    - PACKAGE=xapi-stdext \
       POST_INSTALL_HOOK="opam install alcotest; env TRAVIS=$TRAVIS TRAVIS_JOB_ID=$TRAVIS_JOB_ID bash -ex coverage.sh"
-    - PACKAGE=stdext OCAML_VERSION=4.04.2 REVDEPS=true
-    - PACKAGE=xapi-stdext OCAML_VERSION=4.06.0
+    - PACKAGE=stdext REVDEPS=true
 matrix:
   fast_finish: true
   allow_failures: