Travis: cache local opam switch

Also upgrade opam to 2.0.5.

.travis.yml

@@ -42,6 +42,7 @@ cache:
   - $HOME/travis/.opam
   - $HOME/travis/.esy
   - $HOME/travis/.nvm
+  - ./_opam
   - ./_build/default
 
 install:

test/ci/install-deps.sh

@@ -22,7 +22,7 @@ if [[ $ESY_BUILD == YES ]]; then
   npm --global install [email protected]
 else
   OPAM_RELEASES=https://github.com/ocaml/opam/releases/
-  OPAM_VERSION=2.0.3
+  OPAM_VERSION=2.0.5
 
   case $TRAVIS_OS_NAME in
   "linux") OPAM_OS=linux;;
@@ -37,5 +37,10 @@ else
   sudo chmod a+x /usr/local/bin/opam
 
   opam init -y --bare --disable-sandboxing
-  opam switch create . $OCAML $REPOSITORIES --no-install
+
+  if [ ! -d _opam/bin ]
+  then
+      rm -rf _opam
+      opam switch create . $OCAML $REPOSITORIES --no-install
+  fi
 fi