Fix allow only users with "can edit posts" to reset cache

[jaapmarcus]

Manual clear cache button can be abused if subscribers have access to the wp admin page to manage their profiles for example. To prevent users abusing the cache purge function an potentially causing extra load the button should be removed

[jaapmarcus]

Fixes #8

[jaapmarcus]

For example "User" with no permissions:

2.1.0

vs "2.1.1"

[jakobbouchard]

I didn't think about that! Thanks a lot :)

[jaapmarcus]

I accidentally discovered when an other user changes my permission to subscriber