Added Homebrew.install_gem!

[amancevice]

Useful when gem does not install with executable.

The current Homebrew.install_gem_setup_path! method raises an error when installing a gem that doesn't come with its own executable (e.g. aws-sdk-s3).

I moved the code that installs the gem into Homebrew.install_gem!, And call this method from Homebrew.install_gem_setup_path! to maintain original functionality.

I had to add a make a slight change to add a guard clause so brew style would approve of the changes, but the method is functionally equivalent.

NOTE I didn't add/change tests because these changes should not affect any existing behavior.

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[MikeMcQuaid]

Thanks so much for your first contribution (and for beating me to it)! Without people like you submitting PRs we couldn't run this project. You rock, @amancevice!

[eedwards-sk]

@amancevice Nice!!

Funny because I ran into this too, but I originally just modified the method itself to support a nil exe name, which skipped the 'exe check'. I like your approach, though, to re-use the core behavior but adding an extra check with the 'exe version'.

[amancevice]

Thanks, @eedwards-sk -- I opened a sister PR to install the gem before the sandboxing of brew install: #3949

Your feedback is welcomed!