travis: see if tip is now passing after #44487 fix

golang/go#44487 was likely causing tip to fail with a panic for the go-fuzz travis tests, perhaps due to the `// +build` in slides/crash.go and elsewhere.

Also update the tested versions of Go.

.travis.yml

@@ -7,7 +7,7 @@
 
 language: go
 
-# Test Go 1.11 and 1.12 across linux, osx, windows.
+# Test Go 1.16 and 1.15 across linux, osx, windows.
 # Test Go tip on linux, osx (and explicitly set GO111MODULE=auto for tip, given default might change).
 matrix:
  include:
@@ -17,22 +17,22 @@ matrix:
  - os: linux
  go: tip
  - os: linux
- go: "1.13.x"
+ go: "1.16.x"
  - os: linux
- go: "1.12.x"
+ go: "1.15.x"
  - os: osx
  go: tip
  env: SET_GO111MODULE=1
  - os: osx
  go: tip
  - os: osx
- go: "1.13.x"
+ go: "1.16.x"
  - os: osx
- go: "1.12.x"
+ go: "1.15.x"
  - os: windows
- go: "1.13.x"
+ go: "1.16.x"
  - os: windows
- go: "1.12.x"
+ go: "1.15.x"
 
 # Install coreutils for the 'timeout(1)' utility on windows and osx.
 before_install: