Migrate to circle 2.0

A: This will update our ci stack
B: This might fix a CI bug where circle would pick the wrong version of
ruby

.circleci/config.yml

@@ -0,0 +1,11 @@
+version: 2
+jobs:
+  build:
+    macos:
+      xcode: "8.3.3"
+    steps:
+      - checkout
+      - run: brew update
+      - run: brew install shellcheck
+      - run: shellcheck mac -e SC2039
+      - run: sh mac

circle.yml

@@ -1,14 +0,0 @@
-dependencies:
-  override:
-    - brew install shellcheck
-
-machine:
-  ruby:
-    version: 2.4.1
-  xcode:
-    version: 8.3.3
-
-test:
-  override:
-    - shellcheck mac -e SC2039
-    - sh mac