Remove an existing Boost folder when extracting

scripts/appveyor/install_boost.ps1

@@ -36,6 +36,8 @@ if (Is-LibraryMissing) {
 
   Write-Output "Time taken: $((Get-Date).Subtract($start_time).Seconds) second(s)"
 
+  Remove-Item -Recurse -Force $boostFolder
+
   Write-Output 'Extracting ' + $boostArchive + '...'
   cd
   7z x $boostArchive -o"C:\projects"

scripts/travis/install_boost.sh

@@ -17,6 +17,7 @@ if isLibraryMissing; then
   cd ~
 
   wget https://downloads.sourceforge.net/project/boost/boost/1.61.0/${BOOST_BASENAME}.tar.bz2
+  rm -rf $BOOST_BASENAME
   tar xf ${BOOST_BASENAME}.tar.bz2
 
   cd $BOOST_BASENAME