Update indextank.gemspec

Used pull request code from https://github.com/marbemac/indextank-rb/commit/e2bfd86de3591e6a1e06e7a6dc01deee7c661c8e
Replaced FaradayStack with Faraday Middleware
bumped version to 1.0.13
bumped version
Merge pull request #10 from fcgmedia/master

Replaced Faraday stack with faraday middleware