Update typescript requirement to ^2.8.1

[dependabot-preview[bot]]

Updates the requirements on typescript to permit the latest version.

Release notes

*Sourced from typescript's [releases](https://github.com/Microsoft/TypeScript/releases).* > ## TypeScript 2.8 > For release notes, check out the [release announcement](https://blogs.msdn.microsoft.com/typescript/2018/03/27/announcing-typescript-2-8/) > > For new features, check out the [What's new in TypeScript 2.8](https://github.com/Microsoft/TypeScript/wiki/What's-new-in-TypeScript#typescript-28). > > For breaking changes, check out the [Breaking changes in TypeScript 2.8](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#typescript-28) page. > > For the complete list of fixed issues, check out the issues fixed in [TypeScript 2.8 RC](https://github-redirect.dependabot.com/Microsoft/TypeScript/issues?q=is%3Aissue+milestone%3A%22TypeScript+2.8%22+label%3A%22fixed%22+) and [after TypeScript 2.8 RC](https://github-redirect.dependabot.com/Microsoft/TypeScript/issues?q=is%3Aissue+milestone%3A%22TypeScript+2.8.1%22+label%3A%22fixed%22+). > > Download: > - [npm](https://www.npmjs.com/package/typescript) > - [Visual Studio 2015 plugin](https://www.microsoft.com/en-us/download/details.aspx?id=48593) > - [Visual Studio 2017](https://www.microsoft.com/en-us/download/details.aspx?id=55258) ([Select new version in project options](https://github.com/Microsoft/TypeScript/wiki/Updating-TypeScript-in-Visual-Studio-2017)) > - [NuGet package](https://www.nuget.org/packages/Microsoft.TypeScript.MSBuild/2.8.1) > > Special thanks to all contributors to this release: > > * Aditya Daflapurkar > * Adnan Chowdhury > * Adrian Leonhard > * Alan Agius > * Alex Eagle > * Anders Hejlsberg > * Andrew Casey > * Andy Hanson > * Armando Aguirre > * Benjamin Lichtman > * Daniel Rosenwasser > * Eric Grube > * Esakki Raj > * Herrington Darkholme > * Jack Williams > * Josh Goldberg > * Klaus Meinhardt > * Martyn Janes > * Matt McCutchen > * Mohamed Hegazy > * Nathan Shively-Sanders > * Noj Vek > * Priyantha Lankapura > * Ricardo N Feliciano > * Ron Buckton > * Ryan Cavanaugh > * Sergii Bezliudnyi > * Sheetal Nandi > * Stephan Ginthör > * [**TravCav**](https://github.com/TravCav) > * Wenlu Wang > * Wesley Wigham > * Yuichi Nukiyama > ... (truncated)

Commits

- See full diff in [compare view](https://github.com/Microsoft/TypeScript/commits/v2.8.1)

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[dependabot-preview[bot]]

Superseded by #21.