Build fails with TypeScript 1.8. Pin the typescript version down to 1.7

[vivainio]

TypeScript 1.8 fails with

node_modules/angular2/src/facade/promise.d.ts(1,10): error TS2661: Cannot re-export name that is not defined in the module.

Pinning TypeScript down to 1.7 is a quick fix to make the project build & run

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[vivainio]

@googlebot I signed it!

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[NathanWalker]

@vivainio This PR: https://github.com/angular/angular-electron/pull/10 will actually fix this as well :+1: