Fix macOS build failures (#355)

Summary:
A change in Homebrew has made it complicated to directly install
older versions of CMake. Switch back to the latest for now.

In addition, XCode GM has been released so to continue doing
arm64 builds, we need to use the beta version. More details [here](https://discuss.circleci.com/t/xcode-12-gm-released/37437).

Pull Request resolved: #355

Reviewed By: haozhun

Differential Revision: D23752850

Pulled By: neildhar

fbshipit-source-id: fde05d2135974175d23002a4bd4091c3166225b8

.circleci/config.yml

@@ -41,7 +41,7 @@ workflows:
 # Default settings for Apple jobs (apple-runtime, test-apple-runtime)
 apple_defaults: &apple_defaults
   macos:
-    xcode: "12.0.0"
+    xcode: "12.0.0-beta"
   working_directory: ~/hermes
   environment:
     - TERM: dumb
@@ -196,7 +196,7 @@ jobs:
 
   macos:
     macos:
-      xcode: "12.0.0"
+      xcode: "12.0.0-beta"
     environment:
       - HERMES_WS_DIR: /tmp/hermes
       - TERM: dumb
@@ -207,9 +207,7 @@ jobs:
       - checkout
       - run:
           name: Install dependencies
-          # Force homebrew to install cmake 3.7
-          command: |
-            brew install ninja https://raw.githubusercontent.com/Homebrew/homebrew-core/d25c628ff2510f68c89f4660fc93e86377ae61dc/Formula/cmake.rb
+          command: brew install ninja cmake
       - run:
           name: Set up workspace
           command: |