CI: Switch mainline Nginx to 0.15.0, update older versions

In particular, version 1.11.6 does not even appear in the main
download page. Of course the tarballs are still in the archives,
but not being listed seems like a good criteria to remove it from
the testing matrix. Also, change 1.10.2 to 1.10.3, which is the
latest minor release from the series.

.travis.yml

@@ -6,16 +6,15 @@ compiler:
 
 env:
   # Mainline.
-  - NGINX=1.13.12
-  - NGINX=1.13.12  DYNAMIC=1
+  - NGINX=1.15.0
+  - NGINX=1.15.0  DYNAMIC=1
   # Stable.
   - NGINX=1.14.0
   - NGINX=1.14.0  DYNAMIC=1
   # Other configurations.
   - NGINX=1.12.2
   - NGINX=1.12.2  DYNAMIC=1
-  - NGINX=1.11.6
-  - NGINX=1.10.2
+  - NGINX=1.10.3
   # Version 1.9.15 was the first with loadable module support.
   - NGINX=1.9.15
   - NGINX=1.9.15  DYNAMIC=1