Download the latest milestone version of the eclipse jdtls instead of the unstable snapshot version

[theVerySharpFlat]

These changes address the problems described in #99 where the more unstable "snapshot" version of the eclipse java language server is downloaded and, due to its instability, leads to the language server crashing.

To solve this, I wrote code to parse the milestones page of the jdtls downloads page instead in order to find the url of the latest milestone and download it.