Fix references 'kasen/core' -> 'master'

BUILD_LINUX.md

@@ -1,9 +1,11 @@
 # Build Linux
 
-*Last Updated on April 11, 2020*
+*Last Updated on December 1, 2020*
 
 Please read the [general build guide](BUILD.md) for information on dependencies required for all platforms. Only Linux specific instructions are found in this file.
 
+You can use the [Vircadia Builder](https://github.com/kasenvr/vircadia-builder) to build on Linux more easily. Alternatively, you can follow the manual steps below.
+
 ## Ubuntu 16.04/18.04 specific build guide
 ### Ubuntu 16.04 only
 Add the following line to *.bash_profile*  
@@ -86,7 +88,7 @@ git fetch -a
 
 Then checkout the main branch with:
 ```bash
-git checkout kasen/core
+git checkout master
 ```
 
 ### Using a custom Qt build

hifi_qt.py

@@ -81,7 +81,7 @@ endif()
             else:
                 print("Sorry, " + distro.name(pretty=True) + " is not supported. Please consider helping us out.")
                 print("It's also possible to build Qt for your distribution, please see the documentation at:")
-                print("https://github.com/kasenvr/project-athena/tree/kasen/core/tools/qt-builder")
+                print("https://github.com/kasenvr/project-athena/tree/master/tools/qt-builder")
                 raise Exception('UNKNOWN LINUX VERSION!!!')
         else:
             print("System      : " + platform.system())