Configure CircleCI

* Move off Travis to Circle.
* Run the entire script on macOS infrastructure.
* Performance optimization:
  Enforce a Ruby version to skip Ruby install

We're cheating slightly with this implementation,
but seems worth it for speed of build tradeoff:

* Use pre-installed XCode for compilers, etc.
* Use pre-installed Ruby version.

Related:

https://github.com/thoughtbot/laptop/issues/494

.travis.yml

@@ -1,24 +0,0 @@
-language: bash
-
-# Use container-based infrastructure for quicker build start-up
-sudo: false
-
-addons:
-  apt:
-    sources:
-    - debian-sid # Get shellcheck from the Debian repo
-    packages:
-    - shellcheck
-
-script:
-  - shellcheck mac -e SC2039
-
-matrix:
-  fast_finish: true
-
-notifications:
-  email: false
-
-branches:
-  only:
-    - master

circle.yml

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

mac

@@ -67,7 +67,7 @@ update_shell() {
     fancy_echo "Adding '$shell_path' to /etc/shells"
     sudo sh -c "echo $shell_path >> /etc/shells"
   fi
-  chsh -s "$shell_path"
+  sudo chsh -s "$shell_path" "$USER"
 }
 
 case "$SHELL" in