Remove support for Java 7 on Mac OS X 10.11 and up

[olivierdagenais]

As a follow-on to microsoft/oauth2-useragent#19, update documentation and Homebrew formula to identify Java 8 when running on Mac OS X 10.11 (El Capitan) or greater.

Manual testing

1. On a Mac running OS X 10.10.5 (Yosemite):

   1. Install Homebrew.

   2. Temporarily inject the changes to Library/Formula/git-credential-manager.rb in the Git repo that's cloned as part of installing Homebrew.

   3. Temporarily uninstall Java.

   4. Attempt to install the git-credential-manager formula:

      $ brew install git-credential-manager
      git-credential-manager: Java 1.7+ is required to install this formula.
      You can install with Homebrew Cask:
      brew install Caskroom/cask/java
      
      You can download from:
      http://www.oracle.com/technetwork/java/javase/downloads/index.html
      Error: An unsatisfied requirement failed this build.
      

   5. Install JDK 7u80.

   6. Install the git-credential-manager via Homebrew:

      brew install git-credential-manager
      

   7. Install the GCM in Git:

      git-credential-manager install
      

   8. Confirm the GCM was configured in ~/.gitconfig.

   9. Clone a Git repo hosted in VSTS, clearing any previously-saved credentials if necessary to force the OAuth 2.0 auth flow with the web browser pop-up.

2. On a Mac running OS X 10.11 (El Capitan):

   1. Install Homebrew.

   2. Temporarily inject the changes to Library/Formula/git-credential-manager.rb in the Git repo that's cloned as part of installing Homebrew.

   3. Temporarily uninstall Java.

   4. Attempt to install the git-credential-manager formula:

      $ brew install git-credential-manager
      git-credential-manager: Java 1.8+ is required to install this formula.
      You can install with Homebrew Cask:
      brew install Caskroom/cask/java
      
      You can download from:
      http://www.oracle.com/technetwork/java/javase/downloads/index.html
      Error: An unsatisfied requirement failed this build.
      

   5. Install JDK 8u73.

   6. Install the git-credential-manager via Homebrew:

      brew install git-credential-manager
      

   7. Install the GCM in Git:

      git-credential-manager install
      

   8. Confirm the GCM was configured in ~/.gitconfig.

   9. Clone a Git repo hosted in VSTS, clearing any previously-saved credentials if necessary to force the OAuth 2.0 auth flow with the web browser pop-up.

Mission accomplished!