Fix plugin compatibility with recent Intellij versions

[Latios96]

Hi,

First, thanks for creating this plugin, it's awesome!

Unfortunately, the published version of this plugin is not compatible with recent versions of the IDEs, because of the constrained maximum supported version. This PR fixes that by removing the upper version constraint.

In my opinion, the plugin does not contain anything that will likely change with one of the next Intellij versions. I think it's ok to remove the upper version constraint, so the plugin does not have to be republished for every new Intellij version. This is why I decided to remove the version constraint instead of just bumping it to the most recent version of Intellij.

Besides removing the version constraint, I also did two other changes:

1. I had to update the Gradle Intellij Plugin to be able to use a recent Intellij version, therefore I also needed to update the Gradle wrapper.
2. The IconLoader.getIcon method was deprecated at some point and now also requires a Class to be passed in. Since the deprecated method could be removed at some point, I also fixed this issue.

Let me know what you think about that.

Cheers,
Jan

[justint]

Thanks a bunch Jan, agreed that this is probably the best way to go for building the plugin into the future.

Since the until-build field defaults to intellij.version, this would bring the until-build to 221.*. I think the even better option would be to remove the until-build entirely, since as you mention this plugin isn't likely using anything that will break in the future versions.

I've discovered the updateSinceUntilBuild flag can be turned off which will prevent until-build from being auto-patched by Gradle, allowing the plugin to be installed to all future versions. I'll add a commit to turn that off and merge this in too; again thanks so much for adding in these changes! I'll be glad to get the incompatible plugin issues cleared up for everyone once and for all.

[Latios96]

Oh thats a good point with the until-build default value, I was not aware of that detail..

Thanks for merging and updating the plugin in the market place!