Update dependencies to enable Greenkeeper 🌴 (observablehq#92)

* chore(package): update dependencies

* docs(readme): add Greenkeeper badge

* chore(package): update lockfile yarn.lock

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # @observablehq/stdlib
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/observablehq/stdlib.svg)](https://greenkeeper.io/)
+
 The Observable standard library.
 
 For examples, see https://beta.observablehq.com/@mbostock/standard-library.

package.json

@@ -37,12 +37,12 @@
   },
   "devDependencies": {
     "eslint": "^5.10.0",
-    "husky": "^1.2.0",
-    "jsdom": "^13.0.0",
-    "rollup": "^0.67.4",
-    "rollup-plugin-node-resolve": "^4.0.0",
-    "rollup-plugin-terser": "^3.0.0",
+    "husky": "^2.3.0",
+    "jsdom": "^15.1.0",
+    "rollup": "^1.12.3",
+    "rollup-plugin-node-resolve": "^5.0.0",
+    "rollup-plugin-terser": "^4.0.4",
     "tape": "^4.9.0",
-    "tape-await": "0.1.1"
+    "tape-await": "0.1.2"
   }
 }