Move the Dockerfile to tests.

In practice, docker only allows a single Dockerfile per directory.  This
makes the directory that the Dockerfile is in analogous to a namespace
for your container.  It's important to place these containers in the
correct location.  In this case, I believe that this Dockerfile belongs
in the tests namespace and that we should keep the root project
namespace open for something that potentially "executes" the project in
a more general sense.

Author: Spencer Rinehart

fig.yml

@@ -1,5 +1,5 @@
 build:
-  build: .
+  build: tests
   links:
     - mongo
   volumes: