remove circle.yml, because we have disabled CircleCI.

circle.yml

@@ -1,14 +0,0 @@
-machine:
-  timezone: UTC
-  ruby:
-    version: 2.1.5
-  environment:
-    PROOF: true
-test:
-  override:
-    - script/proof -f
-general:
-  branches:
-    ignore:
-      - gh-pages # no proof script here
-      - master   # don't need to duplicate work