mocha-teamcity-reporter may fail to initialize if 'window' is present

Commit 7071661d checks if 'window' is defined. I do have 'window' available in global scope but I'm not running in mocha-phantomjs. In that case `require('./base')` fails and the reporter does not initialize.

This commit avoids making `require('./base')` mandatory in case 'window' is present.