Just doing rm -rf /home/donald/.dub/packages/libevent-master should do the trick. With recent versions of DUB, its also possible to specify the --force-remove option to let DUB ignore any untracked files (also mentioned as a suggestion in the error message there).

Still the same issue: 

$cd vibenews
$sudo rm -rf ~/.dub/packages/libevent-master
$dub run
Building vibe-d configuration "libevent", build type debug.
Running dmd...
FAIL .dub/packages/vibe-d-0.7.21-alpha.1/.dub/build/libevent-debug-linux.posix-x86_64-dmd-AB0707232CA963B5DA23C2232BBED51B vibe-d staticLibrary
Error executing command run: DMD compile run failed with exit code -9

$cd ~/.dub/packages/vibe-d-0.7.21-alpha.1
$dub run
FAIL .dub/build/libevent-debug-linux.posix-x86_64-dmd-2494B3E0AC5A6FFE1AD0A6AEA442D8E6 vibe-d staticLibrary
Error executing command run: DMD compile run failed with exit code -9

Then I tried to build latest dub: 

git clone github.com/D-Programming-Language/dub.dub
cd dub
./build.sh
Running dmd...
Unable to fork: Cannot allocate memory