[install] Correct dumux-course branch to releases/3.0

scripts/install.sh

@@ -89,19 +89,13 @@ if [ ! -d "dumux" ]; then
 else
     echo "Skip cloning dumux because the folder already exists."
 fi
-cd dumux
-git checkout releases/3.0
-cd ..
 
 # dumux-course
 if [ ! -d "dumux-course" ]; then
-    git clone https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course.git
+    git clone -b releases/3.0 https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course.git
 else
     echo "Skip cloning dumux-course because the folder already exists."
 fi
-cd dumux-course
-git checkout 2018
-cd ..
 
 if [ $? -ne 0 ]; then
     echo "*********************************************************************************************"