chore(travis): set correct version in package.json

The environment variable `RELEASE_BUILD` is used in Jenkins job, when CLI has to be build with strict official version. It's been deleted in a previous commit, which broke this logic - get it back and add comment why we need it.
Also ensure Travis will not push strict version in `npm` by calling `set_package_version` in the travisPack grunt task. In case it is not called, the Travis job, that is executed on push in master/release/release-patch branch, will execute `npm publish` only (as the `travisPack` will not do anything). The prepack script will trigger `grunt prepare` logic, which will not change the version in package.json, so it will remain `5.1.1` for example.
We cannot change the version in the prepack script (i.e. to call `set_package_version` in the `grunt prepare` task, as npm has problem with this approach - it produces `.tgz` of the built package with correct version, however it still tries to publish the version that was originally in the package.json.

Author: rosen-vladimirov

Gruntfile.js

@@ -164,15 +164,23 @@ module.exports = function (grunt) {
 	grunt.loadNpmTasks("grunt-template");
 
 	grunt.registerTask("set_package_version", function (version) {
+		// NOTE: DO NOT call this task in npm's prepack script - it will change the version in package.json,
+		// but npm will still try to publish the version that was originally specified in the package.json/
+		// Also this may break some Jenkins builds as the produced package will have unexpected name.
 		var buildVersion = version !== undefined ? version : buildNumber;
 		if (process.env["BUILD_CAUSE_GHPRBCAUSE"]) {
 			buildVersion = "PR" + buildVersion;
 		}
 
 		var packageJson = grunt.file.readJSON("package.json");
 		var versionParts = packageJson.version.split("-");
-		versionParts[1] = buildVersion;
-		packageJson.version = versionParts.join("-");
+
+		// The env is used in Jenkins job to produce package that will be releasd with "latest" tag in npm (i.e. strict version).
+		if (!process.env["RELEASE_BUILD"]) {
+			versionParts[1] = buildVersion;
+			packageJson.version = versionParts.join("-");
+		}
+
 		grunt.file.write("package.json", JSON.stringify(packageJson, null, "  "));
 	});
 
@@ -221,8 +229,10 @@ module.exports = function (grunt) {
 		if (travis && process.env.TRAVIS_PULL_REQUEST_BRANCH) {
 			return grunt.task.run("pack");
 		}
-
-		console.log(`Skipping travisPack step as the current build is not from PR, so it will be packed from the deploy provider.`);
+		
+		// Set correct version in Travis job, so the deploy will not publish strict version (for example 5.2.0).
+		grunt.task.run("set_package_version");
+		console.log(`Skipping pack step as the current build is not from PR, so it will be packed from the deploy provider.`);
 	});
 	grunt.registerTask("lint", ["tslint:build"]);
 	grunt.registerTask("all", ["clean", "test", "lint"]);