Fixes uncomplete CTools

Ignoring cache/ makes also necessary files ignored:
modules/ctools/page_manager/plugins/cache/page_manager_context.inc
modules/ctools/plugins/cache/simple.inc
modules/ctools/plugins/cache/export_ui.inc

See: https://drupal.org/comment/7001628#comment-7001628
Merge pull request #1012 from notbear/patch-1

Fixes uncomplete CTools by removing overly-broad and irrelevant (?) cache rule