Fix TypeScript types: add a return type to logger methods (#147)

In PR #144, the types for the TypeScript `Logger` interface were added, however
the logger methods are missing return types.  This means TypeScript treats the
return as implicit `any`, and for projects configured to not allow implicit
`any`, that breaks the build.

lib/bugsnag.d.ts

@@ -51,12 +51,12 @@ declare namespace bugsnag {
     }
 
     interface Logger {
-        info();
-        warn();
-        error();
-        debug();
+        info(): void;
+        warn(): void;
+        error(): void;
+        debug(): void;
     }
-    
+
     interface Endpoints {
         notify: string;
         sessions?: string;