[Makefile] Check if Run on Travis CI

I want my Makefile to be used both on local development and Travis CI build. The programming language is Go, and I need to export GOROOT, GOPATH, and PATH during local devlopement. The build environment on Travis CI has these environment variables already set, so I want to check if the running environment is Travis CI or not. If it is, do not export these variables.

I searched the Travis CI environment variables [1], and found that TRAVIS env is set to true in Travis CI build. We can export variables only on Travis CI build as follows:

ifndef TRAVIS
      export GOROOT=$(realpath ../go)
      export GOPATH=$(realpath .)
      export PATH := $(GOROOT)/bin:$(GOPATH)/bin:$(PATH)
endif

ifndef means if not defined, so if TRAVIS is not defined, which means it's local development environment, we will export GOROOT, GOPATH, and PATH.

Tested on Travis CI, Ubuntu Linux 17.04

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