Handle .x version syntax with latest release (#13)

* get latest release for .x syntax version

* added nock as dev dependency

* added test for .x syntax

* updated readme

* updated http client name

* use rest client for getting available versions

* more .x handling

* move nock to setup and teardown

package.json

@@ -29,11 +29,13 @@
   "devDependencies": {
     "@actions/io": "^1.0.0",
     "@types/jest": "^24.0.13",
+    "@types/nock": "^10.0.3",
     "@types/node": "^12.0.4",
     "@types/semver": "^6.0.0",
     "husky": "^2.3.0",
     "jest": "^24.8.0",
     "jest-circus": "^24.7.1",
+    "nock": "^10.0.6",
     "prettier": "^1.17.1",
     "ts-jest": "^24.0.2",
     "typescript": "^3.5.1"