Don't redirect stderr of build_solr_schema

Redirecting stderr to /dev/null makes it difficult to debug errors.
Instead, use `build_solr_schema`'s `-f` option to write the schema to a
file. This shuts up its useless output to stderr even when it succeeds,
so we don't have to redirect.
(cherry picked from commit 367b08b73d77d2c4be9386a892d117e756296664)

Merged #2
Reviewed-by: Antoine Catton <acatton@fusionbox.com>