CI: Update mainline/stable Nginx versions

Only the minor version number is changed, so there is no need to
keep the older versions being tested as well in the test matrix.

.travis.yml

@@ -6,11 +6,11 @@ compiler:
 
 env:
   # Mainline.
-  - NGINX=1.13.4
-  - NGINX=1.13.4  DYNAMIC=1
+  - NGINX=1.13.10
+  - NGINX=1.13.10  DYNAMIC=1
   # Stable.
-  - NGINX=1.12.1
-  - NGINX=1.12.1  DYNAMIC=1
+  - NGINX=1.12.2
+  - NGINX=1.12.2  DYNAMIC=1
   # Other configurations.
   - NGINX=1.11.6
   - NGINX=1.10.2