[openstreetmap-website] Update INSTALL.md (#1173)

[cwbach]

Adds an extra line to load btree-gist extension for database osm_test as well (and not only for database openstreetmap). Gets rid of an migration error with `bundle exec rake test` at the end of the installation.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/1173

-- Commit Summary --

  * Update INSTALL.md

-- File Changes --

    M INSTALL.md (1)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/1173.patch
https://github.com/openstreetmap/openstreetmap-website/pull/1173.diff

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/1173
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