Add additional IntelliJ project file to Git ignore
Add additional IntelliJ project file to Git ignore

This commit adds the IntelliJ ipr extension to .gitignore. These project
files are created when running the Maven goal idea:idea.

brianfrankcooper/YCSB,manolama/YCSB,madhurihn/YCSB_ToyDB,ChristianNavolskyi/YCSB,brianfrankcooper/YCSB,zyguan/ycsb,cricket007/YCSB,leschekhomann/YCSB,madhurihn/YCSB_ToyDB,jaemyoun/YCSB,jaemyoun/YCSB,manolama/YCSB,jaemyoun/YCSB,cricket007/YCSB,brianfrankcooper/YCSB,leschekhomann/YCSB,ChristianNavolskyi/YCSB,brianfrankcooper/YCSB,zyguan/ycsb,zyguan/ycsb,manolama/YCSB,leschekhomann/YCSB,cricket007/YCSB,zyguan/ycsb,ChristianNavolskyi/YCSB,cricket007/YCSB,manolama/YCSB,leschekhomann/YCSB,ChristianNavolskyi/YCSB,jaemyoun/YCSB,madhurihn/YCSB_ToyDB,madhurihn/YCSB_ToyDB