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.

Alternatively this as well:

```ruby
# at the top of the file
require 'semantic/core_ext'

# later...
def get_current_version
  tags = git.semver_tags
  tags.map(:to_version).sort.last
end
```

I actually prefer this approach not only because it does less, but also because it is only converting the strings once, not on every comparison (like I ended up doing on the first pass).