use opam on travis/linux

.travis.yml

@@ -33,6 +33,8 @@ install_linux: &install_linux
       ocaml-native-compilers
       ocaml-findlib
       camlp4
+  - wget https://raw.github.com/ocaml/opam/master/shell/opam_installer.sh -O - | sh -s /usr/local/bin system
+  - opam install sedlex
   # Setup database
   - travis_retry sudo apt-get install mysql-server-5.6 -y
   - mysql -u root -e "create user travis@localhost identified by '';"
@@ -43,7 +45,7 @@ install_linux: &install_linux
   - java -version
   # Build haxe
   - make package_src -s
-  - make -s
+  - opam config exec -- make -s
   - make package_bin -s
   - ls -l out
   - export PATH="$PATH:$TRAVIS_BUILD_DIR"