Switch rspec-example_disabler to a git dependency

Using master until further work on a new rspec-example_disabler
release.

Master contains a fix for an issue in an OpenProject environment
configured with certain private plugins that disable Controller specs
with `RSpec::ExampleDisabler`: rather than being disabled, examples
were instead resulting in a `NoMethodError`.

See: https://github.com/finnlabs/rspec-example_disabler/commit/45cf9ce55ae6a27e74b202c806d30b4865aee03b

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1100 from opf/fix/fix-controller-specs-in-plugin-envs