[ci] Use ruby 2.7 on travis

.travis/before_install.sh

@@ -32,6 +32,7 @@ function install_jdk() {
 }
 
 if travis_isLinux; then
+    change_ruby_version
     gem install bundler
     bundle config set --local path vendor/bundle
     bundle config set --local with release_notes_preprocessing

.travis/common-functions.sh

@@ -57,3 +57,8 @@ function travis_isWindows() {
 function get_pom_version() {
     echo $(./mvnw -q -Dexec.executable="echo" -Dexec.args='${project.version}' --non-recursive org.codehaus.mojo:exec-maven-plugin:3.0.0:exec)
 }
+
+function change_ruby_version() {
+    source "$HOME/.rvm/scripts/rvm"
+    rvm use ruby-2.7
+}

.travis/regression-tester.sh

@@ -1,6 +1,7 @@
 #
 # The functions here require the following scripts:
 # .travis/logger.sh
+# .travis/common-functions.sh
 #
 # The functions here require the following environment variables:
 # PMD_SF_USER
@@ -12,6 +13,7 @@
 # In that case, just a error logging is provided.
 #
 function regression-tester_uploadBaseline() {
+    change_ruby_version
     log_debug "$FUNCNAME branch=${TRAVIS_BRANCH}"
     local targetUrl="https://sourceforge.net/projects/pmd/files/pmd-regression-tester/"
 
@@ -49,6 +51,7 @@ function regression-tester_uploadBaseline() {
 # In that case, just a error logging is provided.
 #
 function regression-tester_executeDanger() {
+    change_ruby_version
     log_debug "$FUNCNAME"
 
     local errexitstate="$(shopt -po errexit)"