[TravisCI] install a patched ptmap for Mac builds

.travis.yml

@@ -133,8 +133,14 @@ install_osx: &install_osx
   - travis_retry brew bundle --file=tests/Brewfile
   - export OPAMYES=1
   - opam init
-  - opam install camlp4 sedlex ocamlfind xml-light extlib rope ptmap
   - eval `opam config env`
+  - opam install camlp4 sedlex ocamlfind xml-light extlib rope
+  # Install a patched ptmap. See https://github.com/backtracking/ptmap/issues/3
+  - opam install obuild qtest
+  - git clone https://github.com/andyli/ptmap.git --branch 4.05 ~/ptmap
+  - pushd ~/ptmap
+  - make && sudo make install
+  - popd
   # check if we need to install neko
   - export REF_CHANGED=1;
   - if [ ! -d "$HOME/neko" ]; then