sort-semver skip numeric identifiers over unsigned long

**What steps did you take:**
If semver identifier in a metadata contains only numeric characters and if that "number" exceeds unsigned long (18,446,744,073,709,551,615), tools simply ignores that version.

```
$ vendir tools sort-semver -v "1.0.0+18446744073709551615"
Versions

Version
1.0.0+18446744073709551615

Highest version: 1.0.0+18446744073709551615

Succeeded

$ vendir tools sort-semver -v "1.0.0+18446744073709551616"
Versions

Version

Succeeded

$ vendir tools sort-semver -v "1.0.0+18446744073709551615x"
Versions

Version
1.0.0+18446744073709551615x

Highest version: 1.0.0+18446744073709551615x

Succeeded

$ vendir tools sort-semver -v "1.0.0+18446744073709551616x"
Versions

Version
1.0.0+18446744073709551616x

Highest version: 1.0.0+18446744073709551616x

Succeeded

```

**What happened:**
Version is ignored

**What did you expect:**
Expecting metadata identifier to work with any alphanumeric characters

**Environment:**

- vendir version 0.23.0
- OS Ubuntu 18.04.3 LTS

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by reproducing the four `vendir tools sort-semver` commands from the issue and locate the `sort-semver` command entry point in the repository. Trace how numeric metadata identifiers are parsed, then add coverage for values above unsigned long and verify that all alphanumeric metadata versions remain in the output.

Written by the indexing model from the issue text.