This commit adds 'latest' version alias that resolves to the
latest available release.
The getPossibleVersions function filters available releases using
startsWith(version), so calling getLatestVersion(version) with an
empty string will return the latest release.
Closes#31
* 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
