build: use tarball instead of git commit for tether drop

It is the only js package that is installed directly from
GitHub - this changes it to download the tarball instead of doing a
git clone.

Background: Git clones against GitHub does currently does not work
with the container build which uses CentOS 6.6. It does not have support for
TLS 1.2 and cannot git clone against GitHub since they removed support for
TLS 1.0 and 1.1.

package.json

@@ -165,7 +165,7 @@
     "rst2html": "github:thoward/rst2html#990cb89",
     "rxjs": "^5.4.3",
     "tether": "^1.4.0",
-    "tether-drop": "https://github.com/torkelo/drop",
+    "tether-drop": "https://github.com/torkelo/drop/tarball/master",
     "tinycolor2": "^1.4.1"
   }
 }