Allow schema loading with an environment variable

Allow tests to use alternate schema paths via an environment variable in
riak_core.

Bump Clique to 0.2.4 so that schema loading is done explicitly by the
caller and not automatically during application startup.

Hi BORS
Merge pull request #692 from basho/bugfix/clique-schema-issues-retry

Allow schema loading with an environment variable

Reviewed-by: jburwell