Tag system image with julia version

[lassepe]

Currently, coc-julia stores a "global" system image in coc-julia-data/sysimg. When the user upgrades to a new julia version and forgets to delete this system image, the server crashes without any informative error. A simple fix may be to store the system image with the corresponding julia version in the file name. This way, coc-julia can automatically detect that there is no valid system image for the current version. This would also make it easier for users to switch between different julia versions.