Unpin chai and eslint versions

greenkeeper.io will let us know if the minor/patch bumps break our code, so go wild.

package.json

@@ -40,8 +40,8 @@
     "test-browser-local": "npm run pre-test-browser && zuul --local 8080 -- test/context-test.js test/scanner-test.js test/parse-test.js test/render-test-browser.js"
   },
   "devDependencies": {
-    "chai": "3.4.0",
-    "eslint": "1.7.3",
+    "chai": "^3.4.0",
+    "eslint": "^1.7.3",
     "mocha": "^2.1.0",
     "zuul": "^3.7.0"
   },