Travis-ci: added support for ppc64le & updated go 1.13 & 1.14 & 1.15

Signed-off-by: Devendranath Thadi <devendranath.thadi3@gmail.com>

.travis.yml

@@ -1,8 +1,11 @@
 language: go
 go:
-  - "1.10.x"
-  - "1.11.x"
-  - "1.12.x"
+  - "1.13.x"
+  - "1.14.x"
+  - "1.15.x"
+arch:
+  - AMD64
+  - ppc64le
 
 before_install:
   - go get github.com/mattn/goveralls