Fix broken link due to googlecode retirement.
Fix broken link due to googlecode retirement.

The previous link to the gtest-1.7.0 zip was broken, making it difficult for a user to follow-along with the documentation.  Project has been moved to github, links and prefixes updated accordingly.

Closes #1688.

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Reviewed-on: https://github.com/bazelbuild/bazel/pull/1688
MOS_MIGRATED_REVID=133572103

davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website