Test Node 6 on Travis

By default, make test requires babel-polyfill and all tests run fine even on
older versions of Node. For fluent-syntax, howeever, we want to also run tests
without the polyfill in order to make sure that fluent-syntax is actually
runnable on Node 6. See projectfluent#164.

.travis.yml

@@ -1,7 +1,10 @@
 sudo: false
 language: node_js
-script: make deps && make dist
+script:
+  - make deps && make dist
+  - [[ $(node --version) = v6.* ]] && make -C fluent-syntax test-without-babel-polyfill
 node_js:
+  - "6"
   - "8"
 cache:
   directories: node_modules

fluent-syntax/makefile

@@ -23,6 +23,10 @@ compat.js: $(SOURCES)
 	    --output.file $@
 	@echo -e " $(OK) $@ built"
 
+.PHONY: test-without-babel-polyfill
+test-without-babel-polyfill:
+	@mocha --recursive --ui tdd --require babel-register test/
+
 clean:
 	@rm -f $(PACKAGE).js compat.js
 	@echo -e " $(OK) clean"