Update nginx version and fix Deploy Button

This upgrades nginx and hard codes a SHA for this release of 1.13.3.

This also sets up `app.json` for the Heroku Deploy Button to use
heroku-buildpack-multi which will pull in other buildpacks from
`.buildpacks`.

[ci skip]

Author: jtgeibel

.buildpacks

@@ -1,5 +1,5 @@
 https://github.com/rcaught/heroku-buildpack-cmake#e4e2c9e
 https://github.com/emk/heroku-buildpack-rust#578d630
 https://codon-buildpacks.s3.amazonaws.com/buildpacks/heroku/emberjs.tgz
-https://github.com/ryandotsmith/nginx-buildpack.git#af813ba
+https://github.com/travis-ci/nginx-buildpack.git#2fbde35
 https://github.com/sgrif/heroku-buildpack-diesel#f605edd

app.json

@@ -56,19 +56,7 @@
   ],
   "buildpacks": [
       {
-        "url": "https://github.com/rcaught/heroku-buildpack-cmake"
+        "url": "https://github.com/heroku/heroku-buildpack-multi"
       },
-      {
-        "url": "https://github.com/emk/heroku-buildpack-rust"
-      },
-      {
-        "url": "https://codon-buildpacks.s3.amazonaws.com/buildpacks/heroku/emberjs.tgz"
-      },
-      {
-        "url": "https://github.com/travis-ci/nginx-buildpack"
-      },
-      {
-        "url": "https://github.com/sgrif/heroku-buildpack-diesel"
-      }
   ]
 }