fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:braces:20180219
- https://snyk.io/vuln/npm:eslint:20180222
- https://snyk.io/vuln/npm:mime:20170907

Latest report for chanzuckerberg/cellxgene:package.json:
https://snyk.io/test/github/chanzuckerberg/cellxgene?targetFile=package.json

package.json

@@ -50,7 +50,7 @@
     "babel-core": "^6.13.2",
     "babel-eslint": "^7.1.1",
     "babel-loader": "^7.1.2",
-    "babel-plugin-istanbul": "^3.1.2",
+    "babel-plugin-istanbul": "^4.0.0",
     "babel-plugin-transform-decorators-legacy": "^1.3.4",
     "babel-plugin-transform-react-constant-elements": "^6.9.1",
     "babel-plugin-transform-runtime": "^6.23.0",
@@ -67,7 +67,7 @@
     "css-loader": "^0.26.1",
     "css-modules-require-hook": "^4.0.1",
     "enzyme": "^2.4.1",
-    "eslint": "^2.10.2",
+    "eslint": "^4.18.2",
     "eslint-loader": "^1.5.0",
     "eslint-plugin-filenames": "^1.1.0",
     "eslint-plugin-import": "^2.2.0",
@@ -89,7 +89,7 @@
     "rimraf": "^2.5.4",
     "style-loader": "^0.13.1",
     "sw-precache-webpack-plugin": "^0.7.1",
-    "url-loader": "^0.5.7",
+    "url-loader": "^0.6.0",
     "web-app-manifest-loader": "^0.1.1",
     "webpack": "^2.2.0-rc.4",
     "webpack-dev-middleware": "^1.6.1",