Remove the versions_to_semver_no_prerelease_idx index

It's not actually being used in the reverse dependencies query and
heroku postgres doesn't like it.

Author: carols10cents

migrations/20170311180634_remove_semver_index/down.sql

@@ -0,0 +1 @@
+CREATE INDEX versions_to_semver_no_prerelease_idx ON versions (crate_id, to_semver_no_prerelease(num) DESC NULLS LAST) WHERE NOT yanked;

migrations/20170311180634_remove_semver_index/up.sql

@@ -0,0 +1 @@
+DROP INDEX versions_to_semver_no_prerelease_idx;