Switch from own docker image to standard selenium images

Since the Selenium images are available for download, and downloading them
is usually faster than building them from scratch, this makes taking
screenshots easier.  Furthermore, since the Selenium image is not specific
to KaTeX, it could as well be used for other purposes, thus saving space
since a single image can be used in multiple projects.

This change also deals with the non-determinism in the Lap screenshot:
We detect the one known (and accepted) alternate rendering and change the
output file name to Lap_alt in this case.  So either Lap or Lap_alt gets
saved to, and if the image is different from both, then one of these files
will show a modification.  On the other hand, if it is equal to either of
these, then the matching one will get overwritten, showing no change.

package.json

@@ -20,6 +20,8 @@
     "jasmine-node": "2.0.0-beta4",
     "jshint": "^2.5.6",
     "less": "~1.7.5",
+    "selenium-webdriver": "^2.46.1",
+    "nomnom": "^1.8.1",
     "uglify-js": "~2.4.15"
   },
   "bin": "cli.js",