Add semantic versioning tagging



Deploy Plan
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Does Infrastructure need to know anything special about this deploy? If so, keep this section and fill it in. **Otherwise, delete it.**

Rollback Plan
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**If this pull request requires anything more complex (e.g., rolling back a migration), you MUST update this section. Otherwise, delete this note.**

To roll back this change, revert the merge with `git revert -m 1 MERGE_SHA` and perform another deploy.

URLs
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* [Jira issue MOBILE-635](https://sportngin.atlassian.net/browse/MOBILE-635)

QA Plan
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Provide a detailed QA plan, or other developers will retain the right to mock you mercilessly.

This could probably be shortened to this:

```ruby
# at the top of the file
require 'semantic/core_ext'

# later...
def get_current_version
  tags = git.semver_tags
  tags.sort |a, b|
    b.to_version <=> a.to_version
  end.first
end
```

That said, I don't know how I feel about this approach.  It will work for a linear release schedule, but if you were to take a project like [rails](https://github.com/rails/rails/releases) for example, you will see that patch versions sometime go out of order for security fixes.

This is probably fine for a first pass, but something to keep in mind.