[CI] Add jenkins jobs for 7.x branch

Author: estolfo

.ci/jobs/elastic+elasticsearch-ruby+7.x.yml

@@ -0,0 +1,11 @@
+---
+- job:
+    name: elastic+elasticsearch-ruby+7.x
+    display-name: 'elastic / elasticsearch-ruby # 7.x'
+    description: Testing the elasticsearch-ruby 7.x branch.
+    parameters:
+    - string:
+        name: branch_specifier
+        default: refs/heads/7.x
+        description: the Git branch specifier to build (<branchName>, <tagName>,
+          <commitId>, etc.)

.ci/test-matrix.yml

@@ -1,7 +1,7 @@
 ---
 ELASTICSEARCH_VERSION:
-- 7.0.0
-- 7.0.1-SNAPSHOT
+- 7.1.0
+- 7.1.0-SNAPSHOT
 
 RUBY_TEST_VERSION:
 - 2.6.1