Fix case on typings file (for build on Linux)

[peterkelly]

When building an app that uses this module on a case-sensitive filesystem, such as those used on most Linux systems, the typings file listed in package.json is not found. The file actually exists, but has the wrong case.

This commit changes the typings to refer to Speech.Browser.Sdk.d.ts, not its lower-cased version, so that it will be found correctly on Linux.

Fixes #72