workaround for typescript 1.1 incompatibility

compilation fails with typescript 1.1 or higher with to the following error
```
$ tools/gup compile
gup  compile
gup    schemas/inputs
gup    shellshape/all
gup      shellshape/extension.js
gup        shellshape/_sources
../src/workspace.ts(434,10): error TS2365: Operator '===' cannot be applied to types 'Window' and 'Window'.
gup      Target `shellshape/extension.js` failed with exit status 1
gup    Target `shellshape/all` failed with exit status 2
gup  Target `compile` failed with exit status 2
```
this patch installs an older typescript version which works correctly
Merge pull request #147 from benalbrecht/patch-1

workaround for typescript 1.1 incompatibility