As a licensed developer, you can download Xcode versions from the Apple Developer Downloads page.
You need Xcode 6.1 to develop using the iOS 8.1 SDK. Xcode 6.1 requires Mac OS X 10.9.4 (Mavericks) or later according to the App Store. The above download page confirms that it requires Mavericks or Yosemite.
If you want to develop using the iOS 8.0 SDK, you will need Xcode 6.0 or later. This requires a mavericks.
You can use Xcode 5.1 for Mountain Lion or Mavericks, but you can only work with the iOS 7.1 SDK (and without Swift).
By the way, from February 15, 2015 you can’t send applications to the store that were developed using the SDK before iOS 8 (i.e. you must use Xcode 6.0 or later at this point):
64-bit and iOS 8 Requirements for application updates
December 17, 2014
As we announced in October, starting February 1, 2015, the new iOS apps introduced on the App Store should include support for the 64-bit version and be built using the iOS 8 SDK. Starting June 1, 2015, application updates will also have to meet the same requirements. To enable a 64-bit project, we recommend that you use the default setting for Xcode "Standard architectures" to create a single binary file with 32-bit and 64-bit code.
See the corresponding developer announcement in the News and Updates page.
So you may want to continue and update it while waiting.