build: migrate from Ubuntu 14.04.5 (Trusty) to 16.04.6 (Xenial) on Travis CI.

Notable changes:
- switch from Oracle JDK 1.8.0_151 to Open JDK 1.8.0_191
  We had to do that because the latter JDK isn't supported anymore
- MySQL has been updated from 5.6.33 to 5.7.25
- Maven has been updated from 3.5.2 to 3.6.0
- shellcheck has been updated from 0.3.3 to 0.3.7

See https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

No functional changes.

Fix #1012

Author: php-coder

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: xenial
 language: java
 
 env:
@@ -65,7 +65,7 @@ deploy:
   skip_cleanup: true
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 services:
   - mysql

src/main/scripts/ci/check-build-and-verify.sh

@@ -156,6 +156,9 @@ if [ "$RUN_ONLY_INTEGRATION_TESTS" = 'no' ]; then
 	print_status "$RFLINT_STATUS" 'Run robot framework lint'
 
 	if [ "$SHELLCHECK_STATUS" != 'skip' ]; then
+		# Shellcheck doesn't support recursive scanning: https://github.com/koalaman/shellcheck/issues/143
+		# Also I don't want to invoke it many times (slower, more code for handling failures), so I prefer this way.
+		# shellcheck disable=SC2207
 		SHELL_FILES=( $(find src/main/scripts -type f -name '*.sh') )
 		shellcheck \
 			--shell bash \