fix(typescript) use gulp-typescript@2.6.0
fix(typescript) use gulp-typescript@2.6.0

Later versions are causing errors. Lock the version for now until
we determine the issue with later versions.

egeland/generator-ng-poly,dustinspecker/generator-ng-poly,TheOneTheOnlyDavidBrown/generator-ng-poly,egeland/generator-ng-poly,ballerabdude/generator-ng-poly,dustinspecker/generator-ng-poly,Ins87/generator-ng-poly,Ins87/generator-ng-poly,ballerabdude/generator-ng-poly,Ins87/generator-ng-poly,dustinspecker/generator-ng-poly,TheOneTheOnlyDavidBrown/generator-ng-poly,egeland/generator-ng-poly,TheOneTheOnlyDavidBrown/generator-ng-poly,ballerabdude/generator-ng-poly