bugfix: missing TS3.1 typings in npm package.

[djcsdy]

Fixes missing TypeScript 3.1 typings in the npm package for moment. Fixes #5486.

Since 2.25.0, package.json specifies:

    "typesVersions": {
        ">=3.1": {
            "*": [
                "ts3.1-typings/*"
            ]
        }
    },

but the ts3.1-typings folder was not included in the npm package.

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 88.468% when pulling 8c627d4 on djcsdy:ts3.1-missing into 36a817d on moment:develop.

[alexturek]

I believe the title needs to begin with [bugfix] (including the brackets)

[alexturek]

Ah - fixed in 6e32433

So I believe the fix is already in, we just need a new release of 2.25

[ichernev]

@alexturek yes, I made the change myself. Thanks @djcsdy!