ADD ARM64 jobs to Travis-CI

[odidev]

Hi Team,

I have raised a PR #56 to add ARM64 support to the Travis-CI.
For AMD64 jobs, Go versions 1.9.x and 1.10.x are used. But ARM64 jobs are failing with these Go versions with below error:

go test: -race and -msan are only supported on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64
The command "go test code.cloudfoundry.org/log-cache-cli/... --race" exited with 2.

However, Go versions 1.13.x, 1.14.x and 1.15.x are working successfully and ARM64 jobs are not failing with the above issue. Hence, I have raised a PR with updated Go versions 1.13.x, 1.14.x and 1.15.x, and Travis-CI is passing well for both AMD64 and ARM64 platforms with the above updated versions.

[cf-gitbot]

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