Doesn't work with TS 1.5 alpha, which is a required dependency

I'm seeing this when compiling TS files:

```
ERROR in [file]
Module build failed: TypeError: Cannot read property 'text' of undefined
at Object.getLength (node_modules\typescript-loader\lib\TypeScriptWebpackHost.js:65:18)
at createLanguageServiceSourceFile (node_modules\typescript-loader\node_modules\typescript\bin\typescript.js:30185:97)
....
```

The issue here seems to be that Typescript's LS host interface has changed between 1.4.1 and 1.5 alpha. typescript-loader probably shouldn't have a required dependency on pre-release versions of typescript; I suggest changing the dependency from ^1.4.1 to ~1.4.1 until TS 1.5 comes out and any required updates to integration with the language services host interface can be made safely.