Switch tests from RHEL 7.5 to 7.6.

(cherry picked from commit 6745ee7cc8)

shippable.yml

@@ -46,7 +46,7 @@ matrix:
     - env: T=network
 
     - env: T=osx/10.11/1
-    - env: T=rhel/7.5/1
+    - env: T=rhel/7.6/1
     - env: T=freebsd/10.4/1
     - env: T=freebsd/11.1/1
     - env: T=linux/centos6/1
@@ -59,7 +59,7 @@ matrix:
     - env: T=linux/ubuntu1604py3/1
 
     - env: T=osx/10.11/2
-    - env: T=rhel/7.5/2
+    - env: T=rhel/7.6/2
     - env: T=freebsd/10.4/2
     - env: T=freebsd/11.1/2
     - env: T=linux/centos6/2
@@ -72,7 +72,7 @@ matrix:
     - env: T=linux/ubuntu1604py3/2
 
     - env: T=osx/10.11/3
-    - env: T=rhel/7.5/3
+    - env: T=rhel/7.6/3
     - env: T=freebsd/10.4/3
     - env: T=freebsd/11.1/3
     - env: T=linux/centos6/3

test/runner/completion/remote.txt

@@ -1,4 +1,4 @@
 freebsd/10.4
 freebsd/11.1
 osx/10.11
-rhel/7.5
+rhel/7.6