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

[Tom Hughes]

Thanks for the contribution.

Having now analysed the problem properly (see https://github.com/openstreetmap/openstreetmap-website/issues/878#issuecomment-193796852 for details) I have pushed a slightly different fix as e1486d79f819a22d72afc6c9d116751315be4db9.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/1173#issuecomment-193798766
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