Bumps js-yaml from 3.12.0 to 3.13.1.

Sourced from js-yaml's changelog.

3.13.1 / 2019-04-05

• Fix possible code execution in (already unsafe) .load(), #480.

3.13.0 / 2019-03-20

• Security fix: safeLoad() can hang when arrays with nested refs
  used as key. Now throws exception for nested arrays. #475.

3.12.2 / 2019-02-26

• Fix noArrayIndent option for root level, #468.

3.12.1 / 2019-01-05

• Added noArrayIndent option, #432.

• 665aadd 3.13.1 released
• da8ecf2 Browser files rebuild
• b2f9e88 Merge pull request #480 from nodeca/toString
• e18afbf Fix possible code execution in (already unsafe) load()
• 9d4ce5e 3.13.0 released
• f64c673 Browser files rebuild
• a567ef3 Restrict data types for object keys
• 59b6e76 Fix test name
• e4267fc 3.12.2 released
• 7231a49 Browser files rebuild
• Additional commits viewable in compare view

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