Test Ruby 2.7 on Travis

Generated code does not yet compile, so set as allow_failure for now
rvm master currently needs to be installed when testing ruby-2.7 on Travis

Tools/travis-linux-install.sh

@@ -95,6 +95,12 @@ case "$SWIGLANG" in
 		travis_retry sudo apt-get -qq install r-base
 		;;
 	"ruby")
+		if [[ "$VER" == "2.7" ]]; then
+			# Ruby 2.7 support is currently only rvm master (30 Dec 2019)
+			travis_retry rvm get master
+			rvm reload
+			rvm list known
+		fi
 		if [[ "$VER" ]]; then
 			travis_retry rvm install $VER
 		fi