Update install swiftlint script to version 0.21.0

scripts/install_swiftlint.sh

@@ -7,7 +7,7 @@
 set -e
 
 SWIFTLINT_PKG_PATH="/tmp/SwiftLint.pkg"
-SWIFTLINT_PKG_URL="https://github.com/realm/SwiftLint/releases/download/0.18.1/SwiftLint.pkg"
+SWIFTLINT_PKG_URL="https://github.com/realm/SwiftLint/releases/download/0.21.0/SwiftLint.pkg"
 
 wget --output-document=$SWIFTLINT_PKG_PATH $SWIFTLINT_PKG_URL