Issue title: GKS: svgplugin.dll: can't load library, error 126 (0x7e)
 Issue body: I get the following error:

`GKS: svgplugin.dll: can't load library, error 126 (0x7e)`

> Windows 10 Pro x64
> Atom 1.24.1 x64
> Julia 0.6.2
> Plots.jl 0.15
> GR.jl 0.28.0

I tried a rebuild of the GR and Plots package, but it did not change anything. Any ideas? Thanks!
 Comments: 
 Comment 0: Did you already:
```
ENV["GRDIR"]=""
Pkg.build("GR")
```
BTW: GR v0.29.0 has just been tagged ...
 Comment 1: Unfortunately that did not help ... :(

With Pkg.checkout("GR") I still stay on v.028.0. Should v.0.29.0 solve my problem?
 Comment 2: No - 0.28.0 should also work - Strange.
Are you using the correct run-time?
```
import GR
GR.version()
```
... should give "0.30.1"
 Comment 3: Yes, thats what I have ...
 Comment 4: 0.30.0 should work, too. I'll have to leave office now but I'll check it this evening ...
 Comment 5: I found a serious bug in the plugin code (on Windows systems). I'll check the new version when the CI has finished.
 Comment 6: GR.jl v0.30.0 is tagged and will be merged soon by the Julia project collaborators. This release will fix the plugin problem.
 Comment 7: v.30.2.post5 solved the problem! Thanks!
{'source': 'GithubIssues', 'repo_name_with_owner': 'jheinen/GR.jl', 'issue_id': 303870803, 'repo_stars': 351.0}