ci: add Go-1.13 (#200)

Author: sbinet

.travis.yml

@@ -1,6 +1,7 @@
 language: go
 
 go:
+  - 1.13.x
   - 1.12.x
   - 1.11.x
   - master

appveyor.yml

@@ -19,7 +19,7 @@ environment:
   #PATH: '%GOPATH%\bin;%CPYTHON2DIR%;%CPYTHON2DIR%\\Scripts;%CPYTHON3DIR%;%CPYTHON3DIR%\\Scripts;C:\msys64\mingw64\bin;C:\msys64\usr\bin\;%PATH%'
   PATH: '%GOPATH%\bin;%CPYTHON3DIR%;%CPYTHON3DIR%\\Scripts;C:\msys64\mingw64\bin;C:\msys64\usr\bin\;%PATH%'
 
-stack: go 1.12
+stack: go 1.13
 
 build_script:
   - python --version