Consolidate version numbers into the env section

.travis.yml

@@ -11,10 +11,10 @@ cache:
     - ${TRAVIS_BUILD_DIR}/examples/browser/node_modules
     - $HOME/.cache/_grpc_gateway_bazel
 before_install:
-  - if [ "${USE_BAZEL}" = true ]; then ./.travis/install-bazel.sh 0.12.0; fi
-  - test "${USE_BAZEL}" = true ||  ./.travis/install-protoc.sh 3.1.0
-  - test "${USE_BAZEL}" = true || ./.travis/install-swagger-codegen.sh 2.2.2
-  - test "${USE_BAZEL}" = true || (nvm install v6.1 && nvm use v6.1 && node --version)
+  - if [ "${USE_BAZEL}" = true ]; then ./.travis/install-bazel.sh $BAZEL_VERSION; fi
+  - test "${USE_BAZEL}" = true ||  ./.travis/install-protoc.sh $PROTOC_VERSION
+  - test "${USE_BAZEL}" = true || ./.travis/install-swagger-codegen.sh $SWAGGER_CODEGEN_VERSION
+  - test "${USE_BAZEL}" = true || (nvm install $NODE_VERSION && nvm use $NODE_VERSION && node --version)
   - test "${USE_BAZEL}" = true || go get github.com/golang/lint/golint
   - test "${USE_BAZEL}" = true || go get github.com/dghubble/sling
   - test "${USE_BAZEL}" = true || go get github.com/go-resty/resty
@@ -68,6 +68,10 @@ env:
   global:
     - "PATH=$PATH:$HOME/local/bin"
     - GO_VERSION_TO_DIFF_TEST="go version go1\.10\.[0-9]\+ linux/amd64"
+    - BAZEL_VERSION=0.12.0
+    - NODE_VERSION=v6.1
+    - PROTOC_VERSION=3.1.0
+    - SWAGGER_CODEGEN_VERSION=2.2.2
   matrix:
     - GATEWAY_PLUGIN_FLAGS=
     - GATEWAY_PLUGIN_FLAGS=request_context=false