Add basic support for --pre

[NickLaMuro]

Description

This is a temporary solution for adding support for pre release based gems. An example for this is the octopress gem:

Which currently only has pre-release gems, and can not be installed in any for currently.

Notes for the future

This is a hack, and I don't really like solving the problem in this way, but it is the simplest for the time being. In reality, we really should move away from a single file installation and move towards a more structured library. I have outlined what I think is required to accomplish this here: https://github.com/sportngin/brew-gem/issues/16

QA

Not the easiest to QA, but here are two possible ways to get this code in place:

1. Modify your brew-gem bin file locally with the changes from this PR. Should be located here: /usr/local/Cellar/brew-gem/CURRENT_VERSION_NUMBER/bin/brew-gem.

2. Check out this branch, make a tar of the bin directory, and install the formula using the following (requires that brew-gem is not installed currently):

   $ git checkout add_basic_support_for_--pre
   $ cd ..
   $ tar --exclude .git -cvf ~/brew-gem-test-for-pr-17.tar.gz brew-gem
   $ shasum ~/brew-gem-test-for-pr-17.tar.gz
   $ curl https://raw.githubusercontent.com/Homebrew/homebrew/master/Library/Formula/brew-gem.rb > brew-gem.rb
   $ sed -i '' "s/url \".*\"/url \"file:\/\/$(ls ~/brew-gem-test-for-pr-17.tar.gz | sed 's/\//\\\//g')\"/" brew-gem.rb
   $ sed -i '' "s/sha1 \".*\"/sha1 \"$(shasum ~/brew-gem-test-for-pr-17.tar.gz | cut -d ' ' -f 1)\"/" brew-gem.rb
   $ brew install brew-gem.rb

Once you have done one of the above, try installing octopress by doing the following:

$ ALLOW_PRE_RELEASE_GEMS=1 brew gem install octopress

[anfleene]

I think this is an ok workaround for now. I agree that we should make this more intuitive going forward.