Makefile, travis target: Only build the site on node.js 4+ now that m…

…etalsmith unsupported older versions.

Makefile

@@ -57,7 +57,10 @@ test-browser: create-html-runners unexpected.js
 travis-chewbacca:
 	./node_modules/.bin/chewbacca --threshold ${CHEWBACCA_THRESHOLD} `echo ${TRAVIS_COMMIT_RANGE} | sed -e 's/\.\.\..*//;'` -- test/benchmark.spec.js
 
-travis: clean lint test travis-chewbacca test-phantomjs test-jasmine test-jest-if-supported-node-version coverage site-build
+travis: clean lint test travis-chewbacca test-phantomjs test-jasmine test-jest-if-supported-node-version coverage
+ifneq ($(shell node --version | grep -vP '^v[0123]\.'),)
+	make site-build
+endif
 	-<coverage/lcov.info ./node_modules/coveralls/bin/coveralls.js
 
 .PHONY: git-dirty-check