Enable running the ui_utils unit-tests on Travis

With the exception of just one test-case, all the current `ui_utils` unit-tests can run successfully on Node.js (since most of them doesn't rely on the DOM).

To get this working, I had to first of all add a new `LIB` build flag such that `gulp lib` produces a `web/pdfjs.js` file that is able to load `pdf.js` successfully.
Second of all, since neither `document` nor `navigator` is available in Node.js, `web/ui_utils.js` was adjusted slightly to avoid errors.

gulpfile.js

@@ -77,6 +77,7 @@ var DEFINES = {
   MINIFIED: false,
   SINGLE_FILE: false,
   COMPONENTS: false,
+  LIB: false,
   PDFJS_NEXT: false,
 };
 
@@ -998,6 +999,7 @@ gulp.task('lib', ['buildnumber'], function () {
     saveComments: false,
     defines: builder.merge(DEFINES, {
       GENERIC: true,
+      LIB: true,
       BUNDLE_VERSION: versionInfo.version,
       BUNDLE_BUILD: versionInfo.commit
     })